Name:	gcbc_DefinedBenefitPlanTargetPlanAssetAllocationsAllowableVariation
Namespace Prefix:	gcbc_
Data Type:	dtr:percentItemType
Balance Type:	na
Period Type:	duration

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:	dtr:percentItemType
Balance Type:	na
Period Type:	duration

Period which an employee's right to exercise an award from a defined contribution plan is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

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

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

Represents the equal matching of employer contribution dollar for dollar.

No definition available.

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

Elements represents the compensation rate of benefit plan which is frozen by the entity.

No definition available.

Name:	gcbc_FrozenPlanCompensationRate
Namespace Prefix:	gcbc_
Data Type:	dtr:percentItemType
Balance Type:	na
Period Type:	duration

Refers to the number of annual installments in which benefit will be paid in the event the executive is entitled to a benefit due to retirement or other termination of employment.

No definition available.

Name:	gcbc_NumberOfAnnualInstallmentsInWhichBenefitWillBePaid
Namespace Prefix:	gcbc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Payment authorized for purchase of additional shares for ESOP.

No definition available.

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

Represents the percentage of employer contribution for next specified percentage.

No definition available.

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

Element presents the period in which lump sum benefit will be paid in the event of death, disability or termination, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

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

Element refers to the Required age to participate in defined benefit pension plan of the entity.

No definition available.

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

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.

No definition available.

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

Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level.

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Amount of net periodic benefit cost (credit) for defined benefit plan.

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Name:	us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

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Name:	us-gaap_DefinedBenefitPlanPlanAssetsTargetAllocationPercentage
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Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

Amount of cost for defined contribution plan.

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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

Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.

No definition available.

Name:	us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

The amount of plan compensation cost recognized during the period.

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

The carrying amount as of the balance sheet date of the obligations recognized for the various benefits provided to former or inactive employees, their beneficiaries, and covered dependents after employment but before retirement.

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

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.

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Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=gcbc_ShortTermAndIntermediateTermSecuritiesMember
Namespace Prefix:
Data Type:	na
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Period Type:

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
Namespace Prefix:
Data Type:	na
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Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
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Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_BalancedFundsMember
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Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
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Name:	us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsByTypeOfDeferredCompensationAxis=gcbc_DeferredBaseSalaryMember
Namespace Prefix:
Data Type:	na
Balance Type:
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Name:	us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsByTypeOfDeferredCompensationAxis=us-gaap_DeferredBonusMember
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