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Securities, Securities in Continuous Unrealized Loss Position (Details) (USD $)
3 Months Ended 9 Months Ended
Mar. 31, 2015
Security
Mar. 31, 2014
Mar. 31, 2015
Security
Mar. 31, 2014
Jun. 30, 2014
Security
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]          
Less Than 12 Months, Fair Value $ 5,259,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue   $ 5,259,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue   $ 9,517,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Less Than 12 Months, Unrealized Losses 62,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss   62,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss   34,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Less Than 12 Months, Number of Securities 5us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear   5us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear   7us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
More Than 12 Months, Fair Value 12,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue   12,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue   19,417,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
More Than 12 Months, Unrealized Losses 1,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss   1,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss   336,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
More Than 12 Months, Number of Securities 1us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear   1us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear   8us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
Total, Fair Value 5,271,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue   5,271,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue   28,934,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Total, Unrealized Losses 63,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss   63,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss   370,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
Total, Number of Securities 6us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1   6us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1   15us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
Other than temporary impairment of available-for-sale security 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsAvailableforsaleSecurities 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsAvailableforsaleSecurities 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsAvailableforsaleSecurities 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsAvailableforsaleSecurities  
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]          
Less Than 12 Months, Fair Value 36,360,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue   36,360,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue   35,243,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Less Than 12 Months, Unrealized Losses 456,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss   456,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss   1,896,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Less Than 12 Months, Number of Securities 33gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear   33gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear   44gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
More Than 12 Months, Fair Value 6,273,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue   6,273,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue   17,656,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
More Than 12 Months, Unrealized Losses 74,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss   74,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss   815,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
More Than 12 Months, Number of Securities 5gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear   5gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear   19gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
Total, Fair Value 42,633,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue   42,633,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue   52,899,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
Total, Unrealized Losses 530,000us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss   530,000us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss   2,711,000us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Total, Number of Securities 38us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions   38us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions   63us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions
Total Securities, Continuous Unrealized Loss Position [Abstract]          
Less Than 12 Months, Fair Value 41,619,000gcbc_SecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue   41,619,000gcbc_SecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue   44,760,000gcbc_SecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Less Than 12 Months, Unrealized Losses 518,000gcbc_SecuritiesContinuousUnrealizedLossPositionAggregateLosses   518,000gcbc_SecuritiesContinuousUnrealizedLossPositionAggregateLosses   1,930,000gcbc_SecuritiesContinuousUnrealizedLossPositionAggregateLosses
Less Than 12 Months, Number of Securities 38gcbc_NumberOfSecuritiesUnrealizedLossPositionLessThanTwelveMonths   38gcbc_NumberOfSecuritiesUnrealizedLossPositionLessThanTwelveMonths   51gcbc_NumberOfSecuritiesUnrealizedLossPositionLessThanTwelveMonths
More Than 12 Months, Fair Value 6,285,000gcbc_SecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue   6,285,000gcbc_SecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue   37,073,000gcbc_SecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
More Than 12 Months, Unrealized Losses 75,000gcbc_SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses   75,000gcbc_SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses   1,151,000gcbc_SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses
More Than 12 Months, Number of Securities 6gcbc_NumberOfSecuritiesUnrealizedLossPositionTwelveMonthsOrLonger   6gcbc_NumberOfSecuritiesUnrealizedLossPositionTwelveMonthsOrLonger   27gcbc_NumberOfSecuritiesUnrealizedLossPositionTwelveMonthsOrLonger
Total, Fair Value 47,904,000gcbc_SecuritiesContinuousUnrealizedLossPositionsFairValue   47,904,000gcbc_SecuritiesContinuousUnrealizedLossPositionsFairValue   81,833,000gcbc_SecuritiesContinuousUnrealizedLossPositionsFairValue
Total, Unrealized Losses 593,000gcbc_SecuritiesContinuousUnrealizedLossPositionsAggregateLosses   593,000gcbc_SecuritiesContinuousUnrealizedLossPositionsAggregateLosses   3,081,000gcbc_SecuritiesContinuousUnrealizedLossPositionsAggregateLosses
Total, Number of Securities 44gcbc_NumberOfSecuritiesUnrealizedLossPosition   44gcbc_NumberOfSecuritiesUnrealizedLossPosition   78gcbc_NumberOfSecuritiesUnrealizedLossPosition
Proceeds from sale of available-for-sale securities, debt 0us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt 0us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt 0us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt 0us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt  
Gross realized gains (losses) on sale of available-for-sale securities 0us-gaap_AvailableForSaleSecuritiesGrossRealizedGains 0us-gaap_AvailableForSaleSecuritiesGrossRealizedGains 0us-gaap_AvailableForSaleSecuritiesGrossRealizedGains 0us-gaap_AvailableForSaleSecuritiesGrossRealizedGains  
Other than temporary impairment of available-for-sale security 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsAvailableforsaleSecurities 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsAvailableforsaleSecurities 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsAvailableforsaleSecurities 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsAvailableforsaleSecurities  
U.S. Government Sponsored Enterprises [Member]          
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]          
Less Than 12 Months, Fair Value 1,970,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
  1,970,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
  1,898,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Less Than 12 Months, Unrealized Losses 30,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
  30,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
  102,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Less Than 12 Months, Number of Securities 1gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
  1gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
  1gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
More Than 12 Months, Fair Value 0us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
  0us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
  0us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
More Than 12 Months, Unrealized Losses 0us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
  0us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
  0us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
More Than 12 Months, Number of Securities 0gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
  0gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
  0gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Total, Fair Value 1,970,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
  1,970,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
  1,898,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Total, Unrealized Losses 30,000us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
  30,000us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
  102,000us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Total, Number of Securities 1us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
  1us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
  1us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions
/ us-gaap_DebtSecurityAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
State and Political Subdivisions [Member]          
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]          
Less Than 12 Months, Fair Value 4,792,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
  4,792,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
  6,693,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Less Than 12 Months, Unrealized Losses 56,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
  56,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
  175,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Less Than 12 Months, Number of Securities 23gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
  23gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
  34gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
More Than 12 Months, Fair Value 79,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
  79,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
  1,815,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
More Than 12 Months, Unrealized Losses 1,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
  1,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
  29,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
More Than 12 Months, Number of Securities 1gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
  1gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
  11gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Total, Fair Value 4,871,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
  4,871,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
  8,508,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Total, Unrealized Losses 57,000us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
  57,000us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
  204,000us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Total, Number of Securities 24us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
  24us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
  45us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Mortgage-backed Securities-residential [Member]          
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]          
Total, Unrealized Losses 0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_MortgageBackedSecuritiesMember
  0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_MortgageBackedSecuritiesMember
  0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_MortgageBackedSecuritiesMember
Mortgage-backed Securities-multi-family [Member]          
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]          
Less Than 12 Months, Fair Value 29,198,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  29,198,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  26,522,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
Less Than 12 Months, Unrealized Losses 368,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  368,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  1,617,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
Less Than 12 Months, Number of Securities 7gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  7gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  7gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
More Than 12 Months, Fair Value 6,044,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  6,044,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  15,440,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
More Than 12 Months, Unrealized Losses 71,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  71,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  764,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
More Than 12 Months, Number of Securities 2gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  2gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  6gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
Total, Fair Value 35,242,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  35,242,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  41,962,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
Total, Unrealized Losses 439,000us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  439,000us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  2,381,000us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
Total, Number of Securities 9us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  9us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  13us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions
/ us-gaap_DebtSecurityAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
Other Securities [Member]          
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]          
Less Than 12 Months, Fair Value 400,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
  400,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
  130,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
Less Than 12 Months, Unrealized Losses 2,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
  2,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
  2,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
Less Than 12 Months, Number of Securities 2gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
  2gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
  2gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
More Than 12 Months, Fair Value 150,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
  150,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
  401,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
More Than 12 Months, Unrealized Losses 2,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
  2,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
  22,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
More Than 12 Months, Number of Securities 2gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
  2gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
  2gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
Total, Fair Value 550,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
  550,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
  531,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
Total, Unrealized Losses 4,000us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
  4,000us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
  24,000us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
Total, Number of Securities 4us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
  4us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
  4us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions
/ us-gaap_DebtSecurityAxis
= us-gaap_OtherDebtSecuritiesMember
Mortgage-backed Securities-residential [Member]          
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]          
Less Than 12 Months, Fair Value 1,684,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
  1,684,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
  4,302,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Less Than 12 Months, Unrealized Losses 14,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
  14,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
  13,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Less Than 12 Months, Number of Securities 1us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
  1us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
  2us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
More Than 12 Months, Fair Value 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
  0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
  0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
More Than 12 Months, Unrealized Losses 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
  0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
  0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
More Than 12 Months, Number of Securities         0us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Total, Fair Value 1,684,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
  1,684,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
  4,302,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Total, Unrealized Losses 14,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
  14,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
  13,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Total, Number of Securities 1us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
  1us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
  2us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Mortgage-backed Securities-multi-family [Member]          
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]          
Less Than 12 Months, Fair Value 2,803,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  2,803,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  4,448,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
Less Than 12 Months, Unrealized Losses 42,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  42,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  5,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
Less Than 12 Months, Number of Securities 2us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  2us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  3us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
More Than 12 Months, Fair Value 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  19,404,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
More Than 12 Months, Unrealized Losses 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  334,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
More Than 12 Months, Number of Securities 0us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  0us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  7us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
Total, Fair Value 2,803,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  2,803,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  23,852,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
Total, Unrealized Losses 42,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  42,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  339,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
Total, Number of Securities 2us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  2us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
  10us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= gcbc_MortgageBackedSecuritiesMultiFamilyMember
Asset-backed Securities [Member]          
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]          
Less Than 12 Months, Fair Value 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
  0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
  0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
Less Than 12 Months, Unrealized Losses 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
  0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
  0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
Less Than 12 Months, Number of Securities 0us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
  0us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
  0us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
More Than 12 Months, Fair Value 12,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
  12,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
  13,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
More Than 12 Months, Unrealized Losses 1,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
  1,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
  2,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
More Than 12 Months, Number of Securities 1us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
  1us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
  1us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
Total, Fair Value 12,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
  12,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
  13,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
Total, Unrealized Losses 1,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
  1,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
  2,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
Total, Number of Securities 1us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
  1us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
  1us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
Corporate Debt Securities [Member]          
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]          
Less Than 12 Months, Fair Value 772,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
  772,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
  767,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Less Than 12 Months, Unrealized Losses 6,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
  6,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
  16,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Less Than 12 Months, Number of Securities 2us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
  2us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
  2us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
More Than 12 Months, Fair Value 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
  0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
  0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
More Than 12 Months, Unrealized Losses 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
  0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
  0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
More Than 12 Months, Number of Securities 0us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
  0us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
  0us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Total, Fair Value 772,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
  772,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
  767,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Total, Unrealized Losses $ 6,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
  $ 6,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
  $ 16,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Total, Number of Securities 2us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
  2us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
  2us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember