A problem with launchers and most programs on wine is that they neglect to "cd" to the correct directory. I usually end up creating a shell script to do it, making it executable, and pointing the launcher to the shell script.
My launcher for Portal on Steam looks like.
To just launch Steam alone, without running a game, you'd leave off the "-applaunch 400 -dxlevel 81". And you'd want to change the home directory to match yours. The "-dxlevel 81" was to fix some slowness I had, but it lowers the quality a bit. To launch a different game, change the 400 to whatever id goes with that game. I think gmod is 4000. A long list of ids is at http://developer.valvesoftware.com/w...pplication_IDs
cd "/home/david/.PlayOnLinux/wineprefix/Steam/drive_c/Program Files/Steam"
env WINEDEBUG=-all WINEPREFIX="/home/david/.PlayOnLinux/wineprefix/Steam" wine Steam.exe -applaunch 400 -dxlevel 81