This guide only covers installation using Lambda Game Utilities and not the manual process. If you wish to do this process manually, you can find plenty of tutorials on the Internet on how to do this
Once Lambda Game Utilities is installed, open the application. You will need to run the application as Administrator. This is required as SteamCMD requires the ability to write files to the application directory.
Click on the green "Manage Game Mounting" button.
Select "Counter-Strike: Source"
Click on "Install Games"
SteamCMD will then go and fetch the files for the Counter-Strike: Source gameserver via SteamCMD. This may take a while depending on your internet speed.
FAQ
To our knowledge, yes. The files are being distributed by Steam itself via it's SteamCMD command line tool. We're only wrapping this in a UI to make the process easier for our players. Lambda Networks does not distribute the files for CSS ourselves. The files are also available through anonymous login.
The files are stored in the following location: [GameDirectory]\lambda_mount\lambda_game_css
If you need to unmount the game files, you can do this by:
Open Steam
Find Garry's Mod
Right Click, go to "Manage" and then "Browse Local Files"
Go to the folder lambda_mount and then delete the folder lambda_game_css