Loans, Paycheck Protection Program (PPP) (Details) - PPP Loans [Member] $ in Millions	12 Months Ended
Jun. 30, 2021 USD ($) Loan	Jun. 30, 2020 USD ($) Loan
PPP Loans [Abstract]
Number of loans \| Loan	1,003	1,267
Loans	$ 62.0	$ 99.8
Fees received from SBA for originating loans	3.5	3.9
Fee income	$ 4.1	$ 0.0

Amount of fees received from Small Business Administration for loan origination under Paycheck Protection Program.

No definition available.

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Namespace Prefix:	gcbc_
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Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

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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
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- 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

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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

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Period Type:	instant

- Details

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