DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Cash and equivalents (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Sep. 30, 2023
Accounting Policies [Abstract]
Cash in non U.S. bank accounts	$ 46,100	$ 45,500

Amount of cash in non-U.S bank accounts as of the reporting date.

No definition available.

Name:	gff_CashInNonUsBankAccounts
Namespace Prefix:	gff_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant