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DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Cash and equivalents (Details) - USD ($) $ in Thousands | Sep. 30, 2025 | Sep. 30, 2024
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Accounting Policies [Abstract] |  |
Cash in non U.S. bank accounts | $ 38,500 | $ 46,100

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- Definition Amount of cash in non-U.S bank accounts as of the reporting date. + References No definition available. + Details
Name: | gff_CashInNonUsBankAccounts
Namespace Prefix: | gff_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_AccountingPoliciesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration