If you mean NFA, it would be interpreted by F-Troop as a prohibition on spending money on background checks and tax-stamp issuance, and used by them to kill all NFA weapons transactions.
The prohibition on possession of untaxed NFA weapons would still be on the books and would be enforced.
If you mean GCA '68, they'd do that same thing: stop issuing mfgr licenses, 4473s, et c. et c., but move against weapons & transactions violative of the prohibitionist elements of the law.