Summary of Significant Accounting Policies, Allowance for Credit Losses on Loans (Details) $ in Thousands	Sep. 30, 2023 USD ($)
Allowance for Loan Losses [Abstract]
Threshold principal amount of nonaccrual loans evaluated individually for impairment	$ 250

The threshold principal amount of nonaccrual loans evaluated individually for impairment.

No definition available.

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