Loans and Allowance for Credit Losses on Loans, Loan Modifications to Borrowers Experiencing Financial Difficulties (Details)	6 Months Ended
Dec. 31, 2023 Loan
Loans and Allowance for Credit Losses on Loans [Abstract]
Number of loans modified to borrowers experiencing financial difficulty	0

Number of financing receivables that have been modified for borrowers experiencing financial difficulty.

No definition available.

Name:	gcbc_FinancingReceivableModificationsLoansToBorrowersExperiencingFinancialDifficultyNumberOfLoans
Namespace Prefix:	gcbc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration