This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons 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.
ПАРОВОЗ, dontgetit, SRP ᴹᴼᴰ | Dark ♥[email protected]!K♥, Joe, [KR] Sky, jeffstuber, Eaglishee, ✘➥IPryse➥✘, Coderz, GamingFlaming, irishboy2316, irishboy2316, DxD one, TuX, Alarm, SiimpL, AIMBOTHACK.NET, Aidan|DaleAGaming, 「MasYa」, 소프넛[언젠간 돌아오겠지], OlDGuY, I put on my robe and wizard hat.