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.
❅Dяαkє Wσlf Bєck❅, nikita0152, Stunrain, Nindota, Chris The Buck, Chris The Buck, agar.iofrind, Mr Wolfsters, Mr. Shrapnel, jonas2002, Š£Ð, scott.thomas7, Pyros, aSeveNtus, mohammadrandomize, drive, bmenini, ze1minuto, White0ut112, Kazitor, Kageyoshi Kuchiki, Juegacubo, Awwent, Paladin Danse, [DRM] BlackFight, Lord of Nightmares, ramigodoysm, CHEZZA1525, SpringTrap(FNAF3), Xigo, Eaz, onion, ★怒号砲☆, TheKhadiro, Tsume, Niiitr0xGaming, Niiitr0xGaming, loruksii, lonely19682005, [Wolfscast]lightwolf25