Instructions
Open the "Garry's Mod" installation directory on your computer, usually in the directory Program Files/Steam/steammapps/Username/garrysmod.
Locate the "cfg" folder and open the "server.cfg."
Find the "sv_defaultgamemode" file and edit it to use whatever game type you want your server to run. For example, if you want to use the game type DarkRP, edit the file to "sv_defaultgamemode DarkRP".