Fair Value Measurements and Fair Value of Financial Instruments, Fair Value, Assets and Liabilities Measured on Nonrecurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Jun. 30, 2019
Assets Measured at Fair Value on Nonrecurring Basis [Abstract]
Impaired loans, recorded investment	$ 3,270	$ 3,894
Impaired loans, related allowance	228	262
Nonrecurring [Member]
Assets Measured at Fair Value on Nonrecurring Basis [Abstract]
Impaired loans, recorded investment	1,809	2,335
Impaired loans, related allowance	228	262
Impaired loans, fair value	1,581	2,073
Foreclosed real estate, recorded investment		53
Foreclosed real estate, related allowance		0
Foreclosed real estate, fair value		53
Nonrecurring [Member] \| Level 1 [Member]
Assets Measured at Fair Value on Nonrecurring Basis [Abstract]
Impaired loans, fair value	0	0
Foreclosed real estate, fair value		0
Nonrecurring [Member] \| Level 2 [Member]
Assets Measured at Fair Value on Nonrecurring Basis [Abstract]
Impaired loans, fair value	0	0
Foreclosed real estate, fair value		0
Nonrecurring [Member] \| Level 3 [Member]
Assets Measured at Fair Value on Nonrecurring Basis [Abstract]
Impaired loans, fair value	$ 1,581	2,073
Foreclosed real estate, fair value		$ 53

This element represents the fair value disclosure of foreclosed real estate.

No definition available.

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

- Definition

Amount of allowance for credit losses related to foreclosed real estate.

+ References

No definition available.

+ Details

Name:	gcbc_ForeclosedRealEstateRelatedAllowance
Namespace Prefix:	gcbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

This element represents the fair value disclosure of impaired loan receivable as presented on the statement of financial position which may include mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts, or any other loans which are due the Company as of the balance sheet date.

+ References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Carrying amount as of the balance sheet date of all assets obtained in full or partial satisfaction of a debt arrangement through foreclosure proceedings or defeasance; includes real and personal property; equity interests in corporations, partnerships, and joint ventures; and beneficial interests in trusts.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5162-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=121593590&loc=d3e4437-111522

+ Details

Name:	us-gaap_ForeclosedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953550-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of allowance for credit losses related to recorded investment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5212-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableRelatedAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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