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Variable Interest Entities - Solar Tax Credit Investments and Related Unfunded Commitments (Details) - USD ($) $ in Thousands | Mar. 31, 2026 | Jun. 30, 2025
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Variable Interest Entities [Abstract] |  |
Gross investment in solar tax credit investments | $ 7,583 | $ 2,586
Accumulated amortization | (7,013) | (2,586)
Net investment in solar tax credit investments | 570 | 0
Unfunded commitments for solar tax credit investments | $ 1,372 | $ 6,381

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

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- Definition The amount of gross investment in solar tax credit investments to provide a return, primarily through the realization of tax benefits. + References No definition available. + Details
Name: | gcbc_SolarTaxCreditInvestmentsGross
Namespace Prefix: | gcbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- 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
- Definition The amount of unfunded commitments for solar tax credit investments related unfunded commitments. + References No definition available. + Details
Name: | gcbc_UnfundedCommitmentsForSolarTaxCreditInvestments
Namespace Prefix: | gcbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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