Employee Benefit Plans, Employee Stock Ownership Plan ("ESOP") (Details) - USD ($) $ in Thousands	12 Months Ended
Jun. 30, 2020	Jun. 30, 2019
Employee Stock Ownership Plan [Abstract]
Payment authorized for purchase of additional shares for ESOP	$ 140	$ 130
ESOP expense	$ 135	$ 127
Unearned shares (in shares)	0	0
Employee Stock Ownership Plan [Member]
Employee Stock Ownership Plan [Abstract]
Award vesting period	3 years

Shares contributed to the ESOP that have not yet been earned.

No definition available.

Name:	gcbc_EmployeeStockOwnershipPlanESOPNumberOfUnearnedShares
Namespace Prefix:	gcbc_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Payment authorized for purchase of additional shares for ESOP.

+ References

No definition available.

+ Details

Name:	gcbc_PaymentAuthorizedForPurchaseOfAdditionalSharesForEsop
Namespace Prefix:	gcbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of plan compensation cost recognized during the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 40
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=109244661&loc=d3e17540-113929

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: