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Deposits (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Jun. 30, 2025 | Jun. 30, 2024
Deposits [Abstract] |  |
Advance payments by borrowers for taxes and insurance | $ 10.1 | $ 10.0
Maximum percentage of broker deposits | 30.00% |
Maximum amount of broker deposits | $ 740.4 |
Brokered deposits | $ 51.6 | $ 0.0

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- Definition Amount of time deposits maturing after twelve months from the balance sheet date. Time deposits include, but are not limited to, certificates of deposits. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References No definition available. + Details
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Namespace Prefix: | gcbc_
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Balance Type: | credit
Period Type: | instant

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- Definition Maximum amount of deposits held by the entity based on policy. + References No definition available. + Details
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Namespace Prefix: | gcbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The maximum percentage of deposits held by the entity. + References No definition available. + Details
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Namespace Prefix: | gcbc_
Data Type: | dtr:percentItemType
Balance Type: | na
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- Definition Carrying amount of accumulated prepayments received by financial institutions from borrowers for taxes (for example, property taxes) and insurance (for example, property and catastrophe) which will periodically be remitted to the appropriate governmental agency or vendor on behalf of the borrower. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 942 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(15)(5)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1 + Details
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Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
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