CS:S Content for GMod:
It is possible to get the CS:S content for free and directly from Steam.
You are going to need Notepad++, steamcmd and about 15 minutes time.
First you have to install and run steamcmd, valve has a simple tutorial for that on their wiki.
Once you have steamcmd up and running enter the following into the console.
login anonymous
app_update 232330
quit
Now open your GMod folder, navigate to the cfg folder inside garrysmod and open mount.cfg with Notepad++.
Uncomment line 9 (remove the //)(, change the path if you need to).
Save the file and you are done.
To update CSS you simply have to re-run steamcmd, you can also automate this by creating a bat file with steamcmd.exe +login anonymous +app_update 232330 +quit inside.