Average amount of investment of financing receivables.

No definition available.

Name:	gcbc_FinancingReceivableAverageRecordedInvestment
Namespace Prefix:	gcbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The percentage of loans guaranteed.

+ References

No definition available.

+ Details

Name:	gcbc_FinancingReceivablesPercentageOfLoansGuaranteed
Namespace Prefix:	gcbc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953423-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=121614798&loc=d3e15032-111544

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953423-111524

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.1)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

+ Details

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

- Definition

Amount, before allowance and fees, of loans and leases receivable. Excludes loans and leases covered under loss sharing agreements.

+ References

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=121593590&loc=d3e4428-111522

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7)
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

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

- Definition

Amount of deferred fees paid by borrowers and unamortized costs incurred to originate loans and leases, unamortized loan commitments and loan syndication fees, and premiums over or discounts from face amounts of loans that are being amortized into income as an adjustment to yield. Excludes amounts for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6378536&loc=d3e10092-111533

+ Details

Name:	us-gaap_UnamortizedLoanCommitmentAndOriginationFeesAndUnamortizedDiscountsOrPremiums
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_ConstructionAndLandMember
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:

- Details

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

- Details

Name:	us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis=gcbc_SmallBusinessAdministrationSBACARESActPaycheckProtectionProgramMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Loans and Allowance for Loan Losses, Major Loan Segments and Classes (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Mar. 31, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019
Major Loan Segments and Classes [Abstract]
Total gross loans	$ 1,090,880	$ 1,090,880		$ 1,012,660
Allowance for loan losses	(19,668)	(19,668)	$ (18,270)	(16,391)	$ (15,205)	$ (13,984)	$ (13,200)
Unearned origination fees and costs, net	(2,714)	(2,714)		(2,747)
Net loans receivable	1,068,498	1,068,498		993,522
Residential Real Estate [Member] \| Residential Real Estate [Member]
Major Loan Segments and Classes [Abstract]
Total gross loans	305,267	305,267		279,332
Allowance for loan losses	(1,985)	(1,985)	(1,998)	(2,091)	(1,864)	(1,470)	(2,026)
Residential Real Estate [Member] \| Construction and Land [Member]
Major Loan Segments and Classes [Abstract]
Total gross loans	8,467	8,467		11,847
Allowance for loan losses	(101)	(101)	(90)	(141)	(101)	(93)	(87)
Residential Real Estate [Member] \| Multi-family [Member]
Major Loan Segments and Classes [Abstract]
Total gross loans	37,470	37,470		25,104
Allowance for loan losses	(314)	(314)	(276)	(176)	(153)	(147)	(180)
Commercial Real Estate [Member] \| Real Estate [Member]
Major Loan Segments and Classes [Abstract]
Total gross loans	454,792	454,792		381,415
Allowance for loan losses	(11,779)	(11,779)	(10,207)	(8,634)	(7,963)	(7,510)	(7,110)
Commercial Real Estate [Member] \| Construction [Member]
Major Loan Segments and Classes [Abstract]
Total gross loans	66,913	66,913		74,920
Allowance for loan losses	(1,941)	(1,941)	(1,847)	(2,053)	(1,822)	(1,467)	(872)
Consumer Loan [Member] \| Home Equity [Member]
Major Loan Segments and Classes [Abstract]
Total gross loans	18,957	18,957		22,106
Allowance for loan losses	(201)	(201)	(265)	(295)	(273)	(270)	(314)
Consumer Loan [Member] \| Consumer Installment [Member]
Major Loan Segments and Classes [Abstract]
Total gross loans	4,499	4,499		4,817
Allowance for loan losses	(157)	(157)	(256)	(197)	(300)	(366)	(250)
Commercial Loans [Member]
Major Loan Segments and Classes [Abstract]
Total gross loans	194,515	194,515		213,119
Allowance for loan losses	(3,190)	$ (3,190)	$ (3,331)	$ (2,804)	$ (2,696)	$ (2,661)	$ (2,361)
Commercial Loans [Member] \| PPP Loans [Member] \| Small Business Administration (SBA) [Member]
Major Loan Segments and Classes [Abstract]
Average loans outstanding	$ 77,600
Percentage of loans guaranteed		100.00%