Loans and Allowance for Loan Losses, Nonaccrual Loans, Interest Income Data (Details) $ in Thousands	3 Months Ended	9 Months Ended
Mar. 31, 2021 USD ($) Loan	Mar. 31, 2020 USD ($)	Mar. 31, 2021 USD ($) Loan	Mar. 31, 2020 USD ($)
Loans and Allowance for Loan Losses [Abstract]
Interest income that would have been recorded if loans had been performing in accordance with original terms	$ 43	$ 64	$ 174	$ 218
Interest income that was recorded on nonaccrual loans	$ 36	$ 51	$ 116	$ 143
Number of loans in nonaccrual that previously participated in loan deferment program due to COVID-19 \| Loan	8		8
Amount of loans in nonaccrual that previously participated in loan deferment program due to COVID-19	$ 248		$ 248

The amount of loans in nonaccrual that previously participated in the loan deferment program due to COVID-19.

No definition available.

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Namespace Prefix:	gcbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The number of loans in nonaccrual that previously participated in the loan deferment program due to COVID-19.

+ References

No definition available.

+ Details

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- Definition

Represents the interest income that was recorded on nonaccrual loans during the period.

+ References

No definition available.

+ Details

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Namespace Prefix:	gcbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

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Balance Type:	na
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- Definition

Reflects the amount of additional interest income that would have been recorded if impaired or nonperforming loans were instead current, in compliance with their original terms, and outstanding throughout the reporting period or since origination (if held for part of the period).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121585226&loc=d3e18794-111554

+ Details

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