Annoyed by the missing content? Well there is a reason for it! Most maps in Garry’s Mod require Counter-Strike: Source as its content is used for the map itself. If you do not have it installed and mounted, you can use the below guide to get the content.
Visit the download page outside of the game, and then follow the below steps:
Step 1. Press the two “Links” that are marked with the red arrows.
Step 2. First go for the “game content”, Press the blue-ish download button, it will re-direct you to another download button.
Step 3. Press this blue-ish download button too, this one will start the download for the “game content” files.
Step 4. Now you may go for the “map files”, Press the download icon in the upper right corner its marked with a red arrow, this will also start the download for the “map files”
Step 5. After you downloaded both the “game content” and the “map files” you go to Steam and right click Garry’s Mod and select “properties”
Step 6. Now you press on “Local Files” and then on “Browse Local Files”
Step 7. Now extract the “CSS Game Content” into the “addons” folder.
Step 8. Aaand now you extract the “CSS Maps” into the “maps folder.
!!! After that, restart your GMOD; join the server again and you should not see any missing textures/errors anymore! At least not due to CS:S