I am not entirely sure, but here's the command line I use with GameCreate:
srcds.exe -game garrysmod -tickrate 33 -norestart -nocrashdialog +ip 126.96.36.199 -port 27015 +maxplayers 6 +map gm_construct -console +exec port27015.cfg
You might want to remove the lan and add the ip.
Also, not sure if you need to add +exec yourcfgfile.cfg
Gamecreate uses port27015, incase I want to use the same ip with different ports, for other servers.
Also in your left4dead folder, make sure you have a file like: update-l4d2.bat
or something similiar. If not create it in notepad.
It should contain:
title GamePackages Left4Dead 2
@HldsUpdateTool.exe -command update -dir . -game left4dead2 -verify_all
Run it and let it update for files if needed.