Loans and Allowance for Loan Losses, Nonaccrual Loans, Interest Income Data (Details) $ in Thousands	3 Months Ended	6 Months Ended
Dec. 31, 2020 USD ($) Loan	Dec. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) Loan	Dec. 31, 2019 USD ($)
Loans and Allowance for Loan Losses [Abstract]
Interest income that would have been recorded if loans had been performing in accordance with original terms	$ 92	$ 53	$ 198	$ 154
Interest income that was recorded on nonaccrual loans	$ 88	$ 42	$ 126	$ 92
Number of loans in nonaccrual that previously participated in loan deferment program due to COVID-19 \| Loan	4		4
Amount of loans in nonaccrual that previously participated in loan deferment program due to COVID-19	$ 204		$ 204

The amount of loans in nonaccrual that previously participated in the loan deferment program due to COVID-19.

No definition available.

Name:	gcbc_FinancingReceivablesAmountOfLoansInNonaccrualThatPreviouslyParticipatedInLoanDefermentProgramDueToCOVID19
Namespace Prefix:	gcbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The number of loans in nonaccrual that previously participated in the loan deferment program due to COVID-19.

+ References

No definition available.

+ Details

Name:	gcbc_FinancingReceivablesNumberOfLoansInNonaccrualThatPreviouslyParticipatedInLoanDefermentProgramDueToCOVID19
Namespace Prefix:	gcbc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the interest income that was recorded on nonaccrual loans during the period.

+ References

No definition available.

+ Details

Name:	gcbc_InterestIncomeThatWasRecordedOnNonaccrualLoansDuringPeriod
Namespace Prefix:	gcbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

Reflects the amount of additional interest income that would have been recorded if impaired or nonperforming loans were instead current, in compliance with their original terms, and outstanding throughout the reporting period or since origination (if held for part of the period).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121585226&loc=d3e18794-111554

+ Details

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