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.
bryanjones74, ✙ Josef Mengele ✙, Demon, [red]폴[blue]츠, letop06, Chloe, ПЗРК, MapleOrnaments[F-B], engi[POL], ✠blue bird✠, TDRR, BlupBlup CSGOEgg.com, QuasarBarkas, Archy Bot, Razorroboham, Thal Verscholen #TeamHeavy, Thal Verscholen #TeamHeavy, Uros Uki, Uros Uki, 감귤 작작먹어, ☜❶☞Ricky Gervais☜❶☞