lol ppl. all u have to do the following steps:
1.Log onto Garry's Mod.
2.Click Start New Game.
3.Choose ur map and set the settings to ur likings.
4.THIS IS IMPORTANT! CHECK THE Local Network Game BOX OR IT WONT WORK.
5.Once u start ur game, type "sv_lan 0" in the console without the quotes. You can enable the console button by clicking options, advanced, and then enable developer console (~)
6.Give ur IP address to ur friends by typing "whats my IP address" into google without quotes.
7. Have fun online!