Name:	gcbc_EmployeeBenefitPlanParticipantContributionAutomaticDeferralRateYearFourAndThereafter
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Data Type:	dtr:percentItemType
Balance Type:	na
Period Type:	duration

Percentage of eligible compensation contributed for participant automatically enrolled in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) during year one.

No definition available.

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

Percentage of eligible compensation contributed for participant automatically enrolled in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) during year three.

No definition available.

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

Percentage of eligible compensation contributed for participant automatically enrolled in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) during year two.

No definition available.

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

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.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)
-SubTopic 205
-Topic 962
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478416/962-205-50-1