Fair Value Measurements and Fair Value of Financial Instruments, Assets and Liabilities Measured on Nonrecurring Basis (Details) $ in Thousands	6 Months Ended
Dec. 31, 2020 USD ($) Approach	Jun. 30, 2020 USD ($)
Assets Measured at Fair Value on Nonrecurring Basis [Abstract]
Impaired loans, recorded investment	$ 2,157	$ 3,270
Impaired loans, related allowance	177	228
Nonrecurring [Member]
Assets Measured at Fair Value on Nonrecurring Basis [Abstract]
Impaired loans, recorded investment	1,250	1,809
Impaired loans, related allowance	177	228
Impaired loans, fair value	1,073	1,581
Foreclosed real estate, recorded investment	385
Foreclosed real estate, related allowance	0
Foreclosed real estate, fair value	$ 385
Nonrecurring [Member] \| Maximum [Member]
Assets Measured at Fair Value on Nonrecurring Basis [Abstract]
Number of approaches used for appraisals \| Approach	3
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	1,073	$ 1,581
Foreclosed real estate, fair value	$ 385

Number of approaches used for appraisal to measure fair value.

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- Definition

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

+ References

No definition available.

+ Details

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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

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Namespace Prefix:	gcbc_
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- 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

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- References

No definition available.

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- 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

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- 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

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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

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