Variable Interest Entities (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Jun. 30, 2024
Variable Interest Entities [Abstract]
Net investment in solar tax credit investments	$ 14	$ 0

Amount after accumulated amortization of solar tax credit investments to provide a return, primarily through the realization of tax benefits.

No definition available.

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