I think most guns simply have expired trademarks or were never trademarked in the first place (i.e. foreign military weapons like various AKs). Something newish like a Kriss Vector probably is and in that case, yeah, they either make a deal with the manufacturer or call it a Vriss Kector. I'd definiteky expect for example H&K to go after game devs, because they hate civilians and they're assholes, but for most other manufacturers it's just free advertising unless the gun sucks in a game, I guess.