1. Download SteamCMD. See this page on how to download it:
https://developer.valvesoftware.com/wiki/SteamCMD

2. Run SteamCMD.

3. In the command prompt that comes up, type and enter the following command.
login anonymous

4. Pick a folder you want to download the content to. It doesn't matter where, we're going to move it. I'm going to just use C:/CSSContent for this guide.
force_install_dir C:/CSSContent

5. Run the following command, and wait for it to download. This will take a hot minute.
app_update 232330

6. When the download is done, navigate to C:/CSSContent

7. Copy the cstrike folder.

8. Navigate to your Garry's Mod installation folder. The folder you in should have folders in it called garrysmod, bin, and sourceengine. Paste in the cstrike folder.

9. Navigate into the garrysmod folder, then from there, the config folder.

10. Open mount.cfg in the text editor of your choice. It should look like this:

11. Below the line that contains "tf", add the following line:
For the big spaces, insert an indent with the TAB button on your keyboard.

12. Save the file.

13. Start Garry's Mod. You now have counter-strike source content installed!