If you are missing CS:S textures in Garry's Mod follow these steps. It will not take long to set up and will fix most of the errors that there are. If you own CS:S, you just need to have the game installed.
- Click Here for the Textures and Map Content. You do need both. This will download the .ZIP file for you.
- Extract the files.
- Open up your file explorer and navigate to your addons folder for Garry's Mod
- Your path will look something like this.
C:\Program Files (x86)\Steam\steamapps\common\GarrysMod\garrysmod\addons
- Paste the extracted files that you downloaded into that addon folder.
- Load up Garry's Mod and you should have all of the necessary textures
If you need any other help, there are a ton of YouTube videos that explain the process.