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.
pose92, thienbaot95, wolfredd66 [Jengibres], pregopracima, Epsilon Mk. 46, MoanFullSpoon69, sadyan.artyom, ENVYUS APEX, xx31315, Ghost1221, therayninja, Al3x is a stupid NIGG3R, ✯✯JJKPLCOM2✯✯, OldskoolGaming, marcinhogamer0202, Temper, «ξαμες», ean.raihan25, Battlefieldkille, Kryporky, SadMan, Triggerhappy454, dmitrifg, Samukambr17, REVTRD, marshalmoosetv, xiongliangtian123, v.david05, Deathahwk443, Frescoia, 卡卡超卡住, jabloniec.jakub, k1ll3r k33mst4r, NoireChan, harrymurray2004, harrymurray2004, ignacio, SAS41, <▒♠_[S]_RaZoR♠▒>, Nopity, KralDeve, christianswed682, PussInBøtts (Olaf), luka0112, gamergr123, 1437481518, B1ts -> Going Back [adding], XXCromiXX, Gnasty, enidan007 +144 others