It's an internet server, just nobody can connect. You have to forward 5 ports for it to work. There's a good guide on portforwarding for Half-Life, the original on portforward.com, which is exactly the same as doing it for Garry's Mod.
Yes, http://www.portforward.com read it.
[L] SandboxMap: gm_constructPlayers: 1 / 15Build: 1909....
-->[L]<-- Sandbox...
I don't think your 30 like your Profile says.
Must've been a glitch. Well, I found my server on the list, but it still says [L] Sandbox.
[L] is normal for a sandbox server, don't pay attention to that.