Press the "Play" button

Step 1:

When you have opened Garry´s Mod, you have to click `Start New Game´



Step 2:

When you have clicked on `Start New Game´, shows you a menu that have the following things:
1: List of maps
2: Numbers of players (max)
3: List of opcional settings
4: A button to start the game


Select the optional settings and the maximun numbers of players

Step 3 (final step):

Cick on `Start Game´ or click two times on the map that you want

Thanks for reading my Epic Guide