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.
furkan_akbas09, in-flix, Henge, paulet932, Lollystok, andresdeleonpaniagua1997, vielvladimir, CrackCocainSpider, wolfburgt, aurimasgarubis123, enzaim3001, Guilmon, krzysko3324, cinek1183, FEPESiK, UncleSHREDDER, scorpion108877, jdhr24, lambo18, MagicalFruitBasket, LazerSpy