I use this everyday to play with friends, other people have probably done this before me and I didn't make this up, i got it from a friend if said friend wants me to delete it, just comment and i'll do it, people who have shown me how to do it later don't count.

First off you have to start Garry's mod, go to singleplayer, and when it sais singleplayer at the top right click the big arse green button and change it to how many players you want to, make sure P2P is checked and it's not a LAN game, then start the game, if done correctly then the game will not pause when you pause the game, a multiplayer world, then invite friends you want to play with and let them join, thats it. GLHF