Loans and Allowance for Credit Losses on Loans - Financial effect of the modifications made to borrowers experiencing financial difficulty (Details)	3 Months Ended	6 Months Ended
Dec. 31, 2024	Dec. 31, 2024
Interest Rate Reduction [Member] \| Commercial Real Estate Portfolio Segment [Member]
Loans and Allowance for Credit Losses on Loans - Financial effect of the modifications made to borrowers experiencing financial difficulty (Details) [Line Items]
Financing Receivable Modifications Weighted Average Interest Rate	1.45%	1.45%

Weighted average interest rate of modified financing receivable.

No definition available.

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Namespace Prefix:	gcbc_
Data Type:	dtr:percentItemType
Balance Type:	na
Period Type:	duration