This download looks like a lua file. You need to place it in your lua folder inside the Garry's Mod folder.
You should end up with a directory structure similar to below:
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
Sevan, DammySumm, [CG]Hippo, SILVΞЯSHIИΞ, BBS|| broken hearted worthless, M0NstR., f122apg, VANTA, Detective Patrick, Festive Taken, Sixten45, GamersMovement.com | Bob Hush, Redogg, Just a ❒, Sir Mjctechguy, Shinycow