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.
lukedavi, m.sap, muhammedalgul_23, Sigalaeff0607, pootis the cat, BatuhanKarayel, misaelbravo19, Binomi89, emreari319ea, mjh9994, MuhammadIzzan_fr_, sshahrabadi92, ivan.sandrin1991, wachis.17, kim231654, SoldierSVK, karapinaregehan, ogili06, Joss SparkleDawg #Zoophobia, KSTN, Taradosaurio Rex, Masamoto - sama, wheresmygui, wheresmygui, Kolbi(HUN), LazerSpy