GOODWILL AND INTANGIBLES - Summary of changes in carrying value of goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
Sep. 30, 2025	Sep. 30, 2024
Goodwill [Roll Forward]
Goodwill, beginning balance	$ 329,393	$ 327,864
Impairment Charges	(136,612)
Foreign currency translation adjustments	(94)	46
Goodwill, ending balance	192,917	329,393
Hunter Fan Company
Goodwill [Roll Forward]
Goodwill from acquisitions		1,483
Pope
Goodwill [Roll Forward]
Goodwill from acquisitions	230
Consumer and Professional Products
Goodwill [Roll Forward]
Goodwill, beginning balance	138,140	136,611
Impairment Charges	(136,612)	(136,612)
Foreign currency translation adjustments	(94)	46
Goodwill, ending balance	1,664	138,140
Consumer and Professional Products \| Hunter Fan Company
Goodwill [Roll Forward]
Goodwill from acquisitions		1,483
Consumer and Professional Products \| Pope
Goodwill [Roll Forward]
Goodwill from acquisitions	230
Home and Building Products
Goodwill [Roll Forward]
Goodwill, beginning balance	191,253	191,253
Impairment Charges	0
Foreign currency translation adjustments	0	0
Goodwill, ending balance	191,253	191,253
Home and Building Products \| Hunter Fan Company
Goodwill [Roll Forward]
Goodwill from acquisitions		$ 0
Home and Building Products \| Pope
Goodwill [Roll Forward]
Goodwill from acquisitions	$ 0

Amount, after accumulated impairment loss, of asset representing future economic benefit arising from other asset acquired in business combination or from joint venture formation or both, that is not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 24
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482548/350-20-55-24

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 805
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479328/805-10-50-8

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 805
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 41
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479303/805-10-55-41

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Topic 805
-SubTopic 740
-Name Accounting Standards Codification
-Section 55
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478064/805-740-55-13

Reference 5: http://www.xbrl.org/2003/role/exampleRef
-Topic 805
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 39
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479303/805-10-55-39

Reference 6: http://www.xbrl.org/2003/role/exampleRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 49
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482785/280-10-55-49

Reference 7: http://www.xbrl.org/2003/role/exampleRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 100
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482078/820-10-55-100

Reference 8: http://www.xbrl.org/2003/role/exampleRef
-Topic 852
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481372/852-10-55-10

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(15))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482598/350-20-45-1

Reference 11: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482573/350-20-50-1

Reference 12: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (h)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482573/350-20-50-1

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(10)(1))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

Reference 14: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 805
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479581/805-30-50-4

+ Details

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

- Definition

Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

+ References

+ Details

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

- Definition

Amount of foreign currency translation gain (loss) which increases (decreases) asset representing future economic benefit from other asset acquired in business combination or from joint venture formation or both, that is not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482573/350-20-50-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 805
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479581/805-30-50-4

+ Details

Name:	us-gaap_GoodwillForeignCurrencyTranslationGainLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of impairment loss from asset representing future economic benefit arising from other asset acquired in business combination or from joint venture formation or both, that is not individually identified and separately recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (b)
-SubTopic 10
-Topic 230
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-28

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 24
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482548/350-20-55-24

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482573/350-20-50-2

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482573/350-20-50-6

Reference 5: http://www.xbrl.org/2003/role/exampleRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 100
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482078/820-10-55-100

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482573/350-20-50-1

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482598/350-20-45-2

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 805
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479581/805-30-50-4

+ Details

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

- Definition

Amount of increase (decrease) from measurement period adjustment of asset representing future economic benefit arising from other asset acquired in business combination or from joint venture formation or both, that is not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (g)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482573/350-20-50-1

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 805
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479581/805-30-50-4

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 805
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 4A
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479907/805-20-50-4A

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 805
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479328/805-10-50-5

Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 805
-SubTopic 10
-Name Accounting Standards Codification
-Section 25
-Paragraph 16
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479405/805-10-25-16

+ Details

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

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_BusinessAcquisitionAxis=gff_HunterFanCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=gff_PopeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReportingUnitAxis=gff_ConsumerAndProfessionalProductsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReportingUnitAxis=gff_HomeandBuildingProductsHBPMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: