Amount of loan modification requests received under a loan deferment program.

No definition available.

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

- Definition

Number of loan modification requests received under a loan deferment program.

+ References

No definition available.

+ Details

Name:	gcbc_FinancingReceivableLoanDeferralProgramNumberOfLoanModificationRequestsReceived
Namespace Prefix:	gcbc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after troubled debt restructuring, of modified financing receivable.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount, before troubled debt restructuring, of financing receivable to be modified.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of financing receivable modified as troubled debt restructuring.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953676-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=121585226&loc=d3e18845-111554

+ Details

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

- Definition

Number of financing receivable contracts modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953682-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Details

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

Loans and Allowance for Loan Losses, Troubled Debt Restructurings (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Dec. 31, 2020 USD ($) Loan Contract	Dec. 31, 2019 Contract	Dec. 31, 2020 USD ($) Loan Contract	Dec. 31, 2019 Contract	Jun. 30, 2020 USD ($) Loan
Troubled Debt Restructurings [Abstract]
Number of contracts \| Contract		0		0
TDR loans which have subsequently defaulted during the period \| Contract	0	0	0	0
Amount of loan modification requests received under loan deferment program	$ 14,500		$ 14,500		$ 193,500
Number of loan modification requests received under loan deferment program \| Loan	66		66		706
Commercial Loans [Member]
Troubled Debt Restructurings [Abstract]
Number of contracts \| Contract			1
Pre-modification outstanding recorded investment			$ 24
Post-modification outstanding recorded investment			24
Current outstanding recorded investment	$ 24		$ 24