Installing a game-mode is a simple process.

• Find and download the game mode you're after
• Login to the game panel at http://gp.pacifices.net, click on the server you want to install the game mode on and click the FTP tab
• Use your favourite FTP client and connect to your Gmod server ( A guide on how to do this can be found here )
• Extract and upload the game mode files to garrysmod/gamemodes
• In the game panel click the Settings tab and change the game mode to the one you just uploaded to your server
• Restart the server