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.
Glovy, Dank_Pool, PimpBlimp5000, lol, Michak89 [PL], GENTLEMEN SPY[ASİL TR], Koloax [Seamus Superadmin], purpleman586, the green dinosaur, PsyKoTMK, Samy aissa el bey, Samy aissa el bey, Samy aissa el bey, das_ist_sparta3, ☣Lord TL☣, ☣Lord TL☣, xboxxxxd, Warpath-the-drone swe, Warpath-the-drone swe, Sokkah Abdelkader