I have Garry's Mod and i started a multiplayer sever on gm_flatgrass and i asked my friend to join but he said that the sever wasent responding. i have no clue what to do all i tried to do was i put sv_lan 0 in the consel menu.
It's not a simple process to host your own server. You'll need to forward the ports for a Source server so your router lets other people see your server.