Fair Value Measurements and Fair Value of Financial Instruments, Assets Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 30, 2019
Assets [Abstract]
Securities available-for-sale	$ 159,263	$ 122,728
Equity securities	251	253
Recurring [Member]
Assets [Abstract]
Securities available-for-sale	159,263	122,728
Equity securities	251	253
Total securities measured at fair value	159,514	122,981
Recurring [Member] \| U.S. Government Sponsored Enterprises [Member]
Assets [Abstract]
Securities available-for-sale	4,545	5,553
Recurring [Member] \| State and Political Subdivisions [Member]
Assets [Abstract]
Securities available-for-sale	120,976	96,570
Recurring [Member] \| Mortgage-backed Securities-Residential [Member]
Assets [Abstract]
Securities available-for-sale	8,621	2,645
Recurring [Member] \| Mortgage-backed Securities-Multi-family [Member]
Assets [Abstract]
Securities available-for-sale	20,578	16,410
Recurring [Member] \| Corporate Debt Securities [Member]
Assets [Abstract]
Securities available-for-sale	4,543	1,550
Recurring [Member] \| Level 1 [Member]
Assets [Abstract]
Securities available-for-sale	4,543	1,550
Equity securities	251	253
Total securities measured at fair value	4,794	1,803
Recurring [Member] \| Level 1 [Member] \| U.S. Government Sponsored Enterprises [Member]
Assets [Abstract]
Securities available-for-sale	0	0
Recurring [Member] \| Level 1 [Member] \| State and Political Subdivisions [Member]
Assets [Abstract]
Securities available-for-sale	0	0
Recurring [Member] \| Level 1 [Member] \| Mortgage-backed Securities-Residential [Member]
Assets [Abstract]
Securities available-for-sale	0	0
Recurring [Member] \| Level 1 [Member] \| Mortgage-backed Securities-Multi-family [Member]
Assets [Abstract]
Securities available-for-sale	0	0
Recurring [Member] \| Level 1 [Member] \| Corporate Debt Securities [Member]
Assets [Abstract]
Securities available-for-sale	4,543	1,550
Recurring [Member] \| Level 2 [Member]
Assets [Abstract]
Securities available-for-sale	154,720	121,178
Equity securities	0	0
Total securities measured at fair value	154,720	121,178
Recurring [Member] \| Level 2 [Member] \| U.S. Government Sponsored Enterprises [Member]
Assets [Abstract]
Securities available-for-sale	4,545	5,553
Recurring [Member] \| Level 2 [Member] \| State and Political Subdivisions [Member]
Assets [Abstract]
Securities available-for-sale	120,976	96,570
Recurring [Member] \| Level 2 [Member] \| Mortgage-backed Securities-Residential [Member]
Assets [Abstract]
Securities available-for-sale	8,621	2,645
Recurring [Member] \| Level 2 [Member] \| Mortgage-backed Securities-Multi-family [Member]
Assets [Abstract]
Securities available-for-sale	20,578	16,410
Recurring [Member] \| Level 2 [Member] \| Corporate Debt Securities [Member]
Assets [Abstract]
Securities available-for-sale	0	0
Recurring [Member] \| Level 3 [Member]
Assets [Abstract]
Securities available-for-sale	0	0
Equity securities	0	0
Total securities measured at fair value	0	0
Recurring [Member] \| Level 3 [Member] \| U.S. Government Sponsored Enterprises [Member]
Assets [Abstract]
Securities available-for-sale	0	0
Recurring [Member] \| Level 3 [Member] \| State and Political Subdivisions [Member]
Assets [Abstract]
Securities available-for-sale	0	0
Recurring [Member] \| Level 3 [Member] \| Mortgage-backed Securities-Residential [Member]
Assets [Abstract]
Securities available-for-sale	0	0
Recurring [Member] \| Level 3 [Member] \| Mortgage-backed Securities-Multi-family [Member]
Assets [Abstract]
Securities available-for-sale	0	0
Recurring [Member] \| Level 3 [Member] \| Corporate Debt Securities [Member]
Assets [Abstract]
Securities available-for-sale	$ 0	$ 0

No definition available.

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

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

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

- Definition

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

+ Details

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

- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=gcbc_MortgageBackedSecuritiesMultiFamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: