Since a lot of people have issues with CSS Textures, here is a dedicated thread, tutorial & board for that.
If you prefer following the video tutorial we also have that.
After downloading the CS:S Content go to your steam, on the Library section you can find your Garry's mod, use the search bar if you have too many games and can't find it.
Right click on it and follow through to Manage > Browse local files.
Afterwards you should find yourself in a folder very similar to that shown on the following screenshot.
Go to the appropriate folder where you'll place your CSS Textures: garrysmod > addons
Now you'll extract your CSS winrar file into the addons folder, it should look like this:
Inside your CSS folder you should have these contents:
That should fix your game! Check out a map that uses CS:S Textures and you should see if it works or not.
If you're still having issues you could always create a thread here and we'll see what we can do to help!
🐲 Devil Yami 🐲
Owner and Founder