Employee Benefit Plans, Defined Contribution Plan (Details) - USD ($) $ in Thousands	12 Months Ended
Jun. 30, 2019	Jun. 30, 2018
Defined Contribution Plan [Abstract]
Required service period to participate in Defined Contribution Plan	3 months
Equal matching of employer contribution dollar for dollar	3.00%
Percentage of employer contribution for next 3%	50.00%
Company contributions associated with the contribution plan	$ 318	$ 247
Maximum [Member]
Defined Contribution Plan [Abstract]
Percentage of compensation allowed to be contributed by employees on pre-tax basis	50.00%

Represents the percentage of compensation allowed to be contributed by employees on a pre-tax basis.

No definition available.

Name:	gcbc_DefinedContributionPlanPercentageOfCompensationAllowedToBeContributedByEmployeeOnPreTaxBasis
Namespace Prefix:	gcbc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the equal matching of employer contribution dollar for dollar.

+ References

No definition available.

+ Details

Name:	gcbc_EqualMatchingOfEmployerContributionDollarForDollar
Namespace Prefix:	gcbc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the percentage of employer contribution for next specified percentage.

+ References

No definition available.

+ Details

Name:	gcbc_PercentageOfEmployerContributionForNextSpecifiedPercentage
Namespace Prefix:	gcbc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Element represents the required service period after which an employee can participate in the defined contribution plan, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	gcbc_RequiredServicePeriodToParticipateInDefinedContributionPlan
Namespace Prefix:	gcbc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

Amount of cost for defined contribution plan.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

+ Details

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

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: