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v3.25.2

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

X
- Definition Amount after accumulated amortization of solar tax credit investments to provide a return, primarily through the realization of tax benefits. + References No definition available. + Details
Name: | gcbc_SolarTaxCreditInvestmentsNet
Namespace Prefix: | gcbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_VariableInterestEntityNotPrimaryBeneficiaryDisclosuresAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration