To be clear, the gun charge is actually a drug charge: it's illegal to own a firearm if you ever use an illegal drug, which was a racist law (1968 Gun Control Act) put in place as a way to go after groups like the Black Panthers, as well as anti-war groups during Vietnam.
Without them knowing that he had used drugs, it would not have been illegal for him to possess the gun.
In fact, a federal judge just ruled that that particular prohibition is unconstitutional on the grounds that there was no 'historical precedent' for it, which is the new test under Bruen.