-Make sure that you have L4D2 INSTALLED !
-In the main menu of GMOD you can see a controller at the bottom right.
-Clic on it and search L4D2.
-Activate it, and then restart your GMOD

If everything is still broken after that, reinstall the map.

And hey, don't ignore the fact that this map is SINGLEPLAYER ONLY, i saw someone who was trash talking about my map bc his materials were broken and plus he was playing with a friend, so before trashtalking, just read everything ffs.

Thanks !