Summary of Significant Accounting Policies, Federal Home Loan Bank Stock (Details) - USD ($) $ in Thousands	12 Months Ended
Jun. 30, 2024	Jun. 30, 2023
Federal Home Loan Bank Stock [Abstract]
Impairment charges	$ 0	$ 0

Amount of other-than-temporary impairment (OTTI) on investment in debt security measured at fair value.

No definition available.

Name:	gcbc_ImpairmentChargesOnDebtSecuritiesAvailableForSale
Namespace Prefix:	gcbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration