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.
Lettgallian, Corvezeo, LIDL, pirinola1123, Zerka [LUA] [DEV], [CRP] SoRaXs #PNJ, Orpheus, Tazmily, Tropical Trail Mix, Cyas / Pussy Slayer, Blujay.exe, daniel1005, Fondateur | Kriax, Sokkah Abdelkader, Jtrinh765