For the GMod 13 Config use Source Engine MP not 2009
This could help mappers
Ill tell you how this works
In Gmod 12 and before we used Source 2009 but it was multiplayer and singleplayer
But now, Gmod 13 thinks singleplayer is multiplayer, explaining the fact that in garrys mod single player when you pause theres disconnect and it does not pause
And theres the line "multiplayer_only" in the GameInfo.txt but it was always like that but it still made me test with Source MP and it worked! So Gmod 13 treats SP like MP and MP like MP.
This problem was introduced in 2012 at the release of GMod 13 for only GMod 13
Happy Mapping! HLModder out.