Summary of Significant Accounting Policies, Income Recognition on Non-performing and Non-accrual loans (Details)	12 Months Ended
Jun. 30, 2024
Income Recognition on Impaired and Nonaccrual loans [Abstract]
Threshold period of delinquency after which a loan is placed on nonaccrual status	90 days

Refers to the threshold period of delinquency after which a loan is placed on nonaccrual status, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	gcbc_ThresholdPeriodOfDelinquencyAfterWhichLoanIsPlacedOnNonaccrualStatus
Namespace Prefix:	gcbc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration