Loans and Allowance for Loan Losses, Paycheck Protection Program (PPP) (Details) - PPP Loans [Member] $ in Millions	3 Months Ended
Dec. 31, 2021 USD ($) Loan	Dec. 31, 2020 USD ($)	Jun. 30, 2021 USD ($) Loan
PPP Loans [Abstract]
Number of loans \| Loan	181		835
Loans	$ 15.0		$ 67.4
Fee income	$ 1.0	$ 1.4

Number of loans Disbursed under the Payment Protection Program (PPP).

No definition available.

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

- References

No definition available.

+ Details

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

- Definition

Income from mortgage banking activities (fees and commissions) and servicing real estate mortgages, credit cards, and other financial assets held by others net of any related impairment of capitalized service costs. Also includes any premiums received in lieu of regular servicing fees on such loans only as earned over the life of the loans.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-K (SK)
-Number 229
-Section 1402
-Subsection Instruction 3

+ Details

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

- Definition

Amount, before allowance and fees, of loans and leases receivable. Excludes 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 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=124259787&loc=d3e4428-111522

+ Details

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

- Details

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