Other Real Estate (ORE) - Narrative (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Real Estate [Line Items]
Other real estate owned net carrying amount	$ 1,900
Outstanding balance	2,500
Allowance for credit losses on other real estate	673	$ 521
Real Estate \| 1- 4 family
Real Estate [Line Items]
Loans in process of foreclosure	$ 1,600

Other Real Estate, Foreclosed Assets, and Repossessed Assets, Allowance For Credit Loss

No definition available.

Name:	fgbi_OtherRealEstateForeclosedAssetsAndRepossessedAssetsAllowanceForCreditLoss
Namespace Prefix:	fgbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Other Real Estate, Foreclosed Assets, and Repossessed Assets, Gross

+ References

No definition available.

+ Details

Name:	fgbi_OtherRealEstateForeclosedAssetsAndRepossessedAssetsGross
Namespace Prefix:	fgbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Other Real Estate, Foreclosed Assets, and Repossessed Assets, Net

+ References

No definition available.

+ Details

Name:	fgbi_OtherRealEstateForeclosedAssetsAndRepossessedAssetsNet
Namespace Prefix:	fgbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Recorded investment of consumer mortgage loan receivables secured by residential real estate properties for which formal foreclosure proceedings are in process.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 35
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL49126937-111524

+ Details

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

- Definition

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.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fgbi_OneToFourFamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: