This is super niche, but for those of you with more than one Garry's Mod player in your household, you may have noticed you can't play on the same server, or can't connect online at all when there's more than one player.

The solution is simple.

Pick one of the computers to do this to, it only has to be done to all computers (different port for each) minus one. One can be left alone.

Open Steam, go to Library, find Garry's Mod, right click and select properties.
From there, you'll find a box for launch options.
Type "+clientport 27006" for the second, 27007 for the third, 27008 for the fourth, and so on.

Tadah! Now you can game w/ your homies or girlfriend or brother or whatever the hell else.