Do a reverse lookup on the IP? This stuff ain't hard. The hard part about this is emulating the server and writing the software to do so. Redirecting requests to a DNS or IP to a locally ran server can be easily manipulated with just the hosts file. Of course some of the funner stuff is gonna require ARP spoofing and other tricks, but yeah.
Nintendo only cares about any sort of possible monetization solely going to them. This is why they go after fan made games that use their assets and videos that aren't used solely for parody or commentary (these are the things covered by free use). Also, you do know that Garry's Mod and VALVe aren't related, right? Gmod has absolutely nothing to do with Steam other than using it as its delivery platform.