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.
daniel28635, mahou shoujo PURUPURURIN, lukedavi, Z4OR0U3Gaming, javiercito, GlAdOs_01001, [PL]Ziomek-Mek, Black Tea za AAAAAA, minecraft gamer pc 64, .:eXon:. PB [HUN] GoodHead #Pyro, kevinjuntonen2, canal108, koutankoutankou1027, poof709, Darkness, 【Firegone】, ayyy lmao, BurninRubber99, Basbas082507, Bot03, CrazyBooze