I don't know perfectly gmod... but i think if you create a garry mod server in that way, it's an only LAN server.
However perhaps a sv_lan 0 in config can solve that.
The best solution, in order to create an internet server, allways in my opinion, is to launch hldsupdatetool with garry mod options ... You can find specific intall for garry mod server here :
http://wiki.garrysmod.com/?title=Dedicated_Server_Setup
In all cases, u need to check if u haven't some firewall blocking some ports (certainly same as HL2DM, 27015 TCP and UDP).