Fair Value Measurements and Fair Value of Financial Instruments (Details) - Liquidation Expenses [Member] - Significant Unobservable Inputs (Level 3) [Member]	Mar. 31, 2025
Platform Operator, Crypto-Asset [Line Items]
Collateral dependent evaluated loans, measurement input	0.45
Foreclosed real estate, measurement input	0.45

Value of input used to measure collateral dependent evaluated loans.

No definition available.

Name:	gcbc_CollateralDependentEvaluatedLoansMeasurementInput
Namespace Prefix:	gcbc_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant