Fair Value Disclosures - Assets measured at fair value on a recurring basis (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Equity securities:
Investment securities available-for-sale, at fair value	$ 27,887	$ 31,302
Residential mortgage-backed - US agency and GSEs
Equity securities:
Investment securities available-for-sale, at fair value	20	20
Corporate bonds
Equity securities:
Investment securities available-for-sale, at fair value	11,947	15,047
Recurring
Equity securities:
Total investment securities	28,275	31,663
Recurring \| Residential mortgage-backed - US agency and GSEs
Equity securities:
Investment securities available-for-sale, at fair value	20	20
Recurring \| Corporate bonds
Equity securities:
Investment securities available-for-sale, at fair value	11,947	15,047
Recurring \| Municipal bonds
Equity securities:
Investment securities available-for-sale, at fair value	15,920	16,235
Recurring \| Large cap equity mutual fund
Equity securities:
Equity investment securities, at fair value	52	45
Recurring \| Other mutual funds
Equity securities:
Equity investment securities, at fair value	336	316
Level 1 \| Recurring
Equity securities:
Total investment securities	388	361
Level 1 \| Recurring \| Large cap equity mutual fund
Equity securities:
Equity investment securities, at fair value	52	45
Level 1 \| Recurring \| Other mutual funds
Equity securities:
Equity investment securities, at fair value	336	316
Level 2 \| Recurring
Equity securities:
Total investment securities	26,419	29,827
Level 2 \| Recurring \| Residential mortgage-backed - US agency and GSEs
Equity securities:
Investment securities available-for-sale, at fair value	20	20
Level 2 \| Recurring \| Corporate bonds
Equity securities:
Investment securities available-for-sale, at fair value	11,947	15,047
Level 2 \| Recurring \| Municipal bonds
Equity securities:
Investment securities available-for-sale, at fair value	14,452	14,760
Level 3 \| Recurring
Equity securities:
Total investment securities	1,468	1,475
Level 3 \| Recurring \| Municipal bonds
Equity securities:
Investment securities available-for-sale, at fair value	$ 1,468	$ 1,475

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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479130/326-30-45-1

+ Details

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

- References

No definition available.

+ Details

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

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(2))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483467/210-10-45-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 1A
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482736/825-10-45-1A

+ 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
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MunicipalBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=gbny_LargeCapEquityMutualFundMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=gbny_OtherMutualFundsMember
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: