The threshold principal amount of nonaccrual loans evaluated individually for impairment.

No definition available.

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- Definition

Period between issuance and maturity of loan, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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- Definition

The ratio of a loan's size to the value of the property, which secures the loan.

+ References

No definition available.

+ Details

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- Definition

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.

+ References

No definition available.

+ Details

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- Definition

Amount charged to advertising expense for the period, which are expenses incurred with the objective of increasing revenue for a specified brand, product or product line.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 720
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- Definition

Amount of allowance for credit loss for debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-Topic 326
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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Threshold period when financing receivable is considered past due to write off as uncollectible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 14
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-14

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- Definition

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
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Balance Type:	na
Period Type:	duration

- Definition

Represents the number of stores.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
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- Definition

Useful life of property, plant, and equipment, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, land and land improvement; building; machinery and equipment; furniture and fixture; and work of art, historical treasure, or similar asset classified as collection.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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- Details

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- Details

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- Details

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Period Type:

- Details

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Balance Type:
Period Type:

- Details

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- Details

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Period Type:

- Details

Name:	srt_StatementScenarioAxis=gcbc_CompanyHoldsTheFirstMortgageMember
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- Details

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Summary of significant accounting policies (Details) $ in Thousands	12 Months Ended
	Jun. 30, 2026 USD ($) Office Segment	Jun. 30, 2025 USD ($)	Jun. 30, 2024 USD ($)
Summary of significant accounting policies [Abstract]
Number of offices (in Office) \| Office	19
Number of reportable segments (in Segment) \| Segment	1
Allowance for credit losses on securities held-to-maturity (in Dollars)	$ 530	$ 548	$ 483
Threshold period of delinquency after which a loan is placed on nonaccrual status	90 days
Threshold principal amount of nonaccrual loans evaluated individually for impairment (in Dollars)	$ 250
Advertising expense (in Dollars)	506	486
U.S. Treasury Securities [Member]
Summary of significant accounting policies [Abstract]
Allowance for credit losses on securities held-to-maturity (in Dollars)	$ 0	$ 0
Maximum [Member]
Summary of significant accounting policies [Abstract]
Accrued interest receivable threshold period for past due write off	120 days
Maximum [Member] \| Building and Improvements [Member]
Summary of significant accounting policies [Abstract]
Property, plant and equipment, useful life	50 years
Maximum [Member] \| Furniture and Equipment [Member]
Summary of significant accounting policies [Abstract]
Property, plant and equipment, useful life	10 years
Maximum [Member] \| Commercial Portfolio [Member]
Summary of significant accounting policies [Abstract]
Loan term	20 years
Minimum [Member]
Summary of significant accounting policies [Abstract]
Accrued interest receivable threshold period for past due write off	90 days
Minimum [Member] \| Building and Improvements [Member]
Summary of significant accounting policies [Abstract]
Property, plant and equipment, useful life	15 years
Minimum [Member] \| Furniture and Equipment [Member]
Summary of significant accounting policies [Abstract]
Property, plant and equipment, useful life	3 years
Home Equity [Member] \| Maximum [Member]
Summary of significant accounting policies [Abstract]
Loan term	25 years
Home Equity [Member] \| Maximum [Member] \| Company Holds the First Mortgage [Member]
Summary of significant accounting policies [Abstract]
Loan-to-value ratio	85.00%
Home Equity [Member] \| Maximum [Member] \| Company does not Hold the First Mortgage [Member]
Summary of significant accounting policies [Abstract]
Loan-to-value ratio	65.00%