Loans and Allowance for Loan Losses, Impaired Loans By Loan Portfolio Class (Details) - USD ($)	3 Months Ended	9 Months Ended
Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021	Jun. 30, 2021
Loans and Allowance for Loan Losses [Abstract]
Threshold principal amount of nonaccrual loans evaluated individually for impairment	$ 100,000		$ 100,000
With no related allowance recorded [Abstract]
Recorded investment	1,151,000		1,151,000		$ 970,000
Unpaid principal	1,151,000		1,151,000		970,000
Average recorded investment	1,271,000	$ 1,023,000	1,225,000	$ 1,090,000
Interest income recognized	8,000	1,000	28,000	22,000
With an allowance recorded [Abstract]
Recorded investment	9,734,000		9,734,000		5,325,000
Unpaid principal	9,734,000		9,734,000		5,325,000
Related allowance	2,123,000		2,123,000		391,000
Average recorded investment	7,781,000	2,368,000	6,469,000	1,953,000
Interest income recognized	118,000	89,000	262,000	149,000
Total impaired [Abstract]
Recorded investment	10,885,000		10,885,000		6,295,000
Unpaid principal	10,885,000		10,885,000		6,295,000
Related allowance	2,123,000		2,123,000		391,000
Average recorded investment	9,052,000	3,391,000	7,694,000	3,043,000
Interest income recognized	126,000	90,000	290,000	171,000
Residential Real Estate [Member] \| Residential Real Estate [Member]
With no related allowance recorded [Abstract]
Recorded investment	929,000		929,000		370,000
Unpaid principal	929,000		929,000		370,000
Average recorded investment	846,000	381,000	577,000	392,000
Interest income recognized	3,000	1,000	13,000	11,000
With an allowance recorded [Abstract]
Recorded investment	2,064,000		2,064,000		723,000
Unpaid principal	2,064,000		2,064,000		723,000
Related allowance	601,000		601,000		103,000
Average recorded investment	2,159,000	710,000	1,612,000	1,053,000
Interest income recognized	10,000	2,000	43,000	27,000
Total impaired [Abstract]
Recorded investment	2,993,000		2,993,000		1,093,000
Unpaid principal	2,993,000		2,993,000		1,093,000
Related allowance	601,000		601,000		103,000
Average recorded investment	3,005,000	1,091,000	2,189,000	1,445,000
Interest income recognized	13,000	3,000	56,000	38,000
Residential Real Estate [Member] \| Multi-family [Member]
With no related allowance recorded [Abstract]
Recorded investment					0
Unpaid principal					0
Average recorded investment		0		40,000
Interest income recognized		0		0
With an allowance recorded [Abstract]
Related allowance					0
Total impaired [Abstract]
Recorded investment					0
Unpaid principal					0
Related allowance					0
Average recorded investment		0		40,000
Interest income recognized		0		0
Commercial Real Estate [Member] \| Real Estate [Member]
With no related allowance recorded [Abstract]
Recorded investment	70,000		70,000		281,000
Unpaid principal	70,000		70,000		281,000
Average recorded investment	71,000	307,000	351,000	321,000
Interest income recognized	1,000	0	9,000	3,000
With an allowance recorded [Abstract]
Recorded investment	3,721,000		3,721,000		945,000
Unpaid principal	3,721,000		3,721,000		945,000
Related allowance	1,147,000		1,147,000		58,000
Average recorded investment	1,864,000	0	1,085,000	0
Interest income recognized	75,000	0	93,000	0
Total impaired [Abstract]
Recorded investment	3,791,000		3,791,000		1,226,000
Unpaid principal	3,791,000		3,791,000		1,226,000
Related allowance	1,147,000		1,147,000		58,000
Average recorded investment	1,935,000	307,000	1,436,000	321,000
Interest income recognized	76,000	0	102,000	3,000
Commercial Real Estate [Member] \| Construction [Member]
With an allowance recorded [Abstract]
Recorded investment	102,000		102,000		102,000
Unpaid principal	102,000		102,000		102,000
Related allowance	1,000		1,000		1,000
Average recorded investment	102,000	102,000	102,000	102,000
Interest income recognized	0	0	0	0
Total impaired [Abstract]
Recorded investment	102,000		102,000		102,000
Unpaid principal	102,000		102,000		102,000
Related allowance	1,000		1,000		1,000
Average recorded investment	102,000	102,000	102,000	102,000
Interest income recognized	0	0	0	0
Consumer Loan [Member] \| Home Equity [Member]
With no related allowance recorded [Abstract]
Recorded investment	128,000		128,000		224,000
Unpaid principal	128,000		128,000		224,000
Average recorded investment	128,000	230,000	128,000	173,000
Interest income recognized	0	0	0	0
With an allowance recorded [Abstract]
Recorded investment	320,000		320,000		321,000
Unpaid principal	320,000		320,000		321,000
Related allowance	45,000		45,000		73,000
Average recorded investment	320,000	321,000	321,000	381,000
Interest income recognized	4,000	2,000	10,000	12,000
Total impaired [Abstract]
Recorded investment	448,000		448,000		545,000
Unpaid principal	448,000		448,000		545,000
Related allowance	45,000		45,000		73,000
Average recorded investment	448,000	551,000	449,000	554,000
Interest income recognized	4,000	2,000	10,000	12,000
Consumer Loan [Member] \| Consumer Installment [Member]
With no related allowance recorded [Abstract]
Recorded investment	5,000		5,000
Unpaid principal	5,000		5,000
Average recorded investment	2,000		1,000
Interest income recognized	1,000		1,000
With an allowance recorded [Abstract]
Related allowance	0		0
Total impaired [Abstract]
Recorded investment	5,000		5,000
Unpaid principal	5,000		5,000
Related allowance	0		0
Average recorded investment	2,000		1,000
Interest income recognized	1,000		1,000
Commercial Loans [Member]
With no related allowance recorded [Abstract]
Recorded investment	19,000		19,000		95,000
Unpaid principal	19,000		19,000		95,000
Average recorded investment	224,000	105,000	168,000	164,000
Interest income recognized	3,000	0	5,000	8,000
With an allowance recorded [Abstract]
Recorded investment	3,527,000		3,527,000		3,234,000
Unpaid principal	3,527,000		3,527,000		3,234,000
Related allowance	329,000		329,000		156,000
Average recorded investment	3,336,000	1,235,000	3,349,000	417,000
Interest income recognized	29,000	85,000	116,000	110,000
Total impaired [Abstract]
Recorded investment	3,546,000		3,546,000		3,329,000
Unpaid principal	3,546,000		3,546,000		3,329,000
Related allowance	329,000		329,000		$ 156,000
Average recorded investment	3,560,000	1,340,000	3,517,000	581,000
Interest income recognized	$ 32,000	$ 85,000	$ 121,000	$ 118,000

The threshold principal amount of nonaccrual loans evaluated individually for impairment.

No definition available.

Name:	gcbc_FinancingReceivableThresholdPrincipalAmountOfNonaccrualLoansEvaluatedIndividuallyForImpairment
Namespace Prefix:	gcbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	gcbc_TotalImpairedAbstract
Namespace Prefix:	gcbc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	gcbc_WithAnAllowanceRecordedAbstract
Namespace Prefix:	gcbc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	gcbc_WithNoRelatedAllowanceRecordedAbstract
Namespace Prefix:	gcbc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableGrossAllowanceAndNetAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=d3e5212-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=123577603&loc=d3e5258-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of interest income recognized on a cash-basis method of accounting that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953550-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of allowance for credit losses related to recorded investment.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableRelatedAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of average investment of impaired financing receivables for which there is no related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=123577603&loc=d3e5258-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=d3e5212-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of interest income, recognized on cash basis, on financing receivables, for which there is no related allowance for credit losses and are recognized as impaired.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeCashBasisMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of unpaid principal balance of impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of average investment of impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of interest income on financing receivables, for which there is a related allowance for credit losses, recognized on a cash-basis method of accounting that the financing receivables were impaired.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeCashBasisMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Unpaid principal balance of impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=gcbc_MultiFamilyLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_RealEstateLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: