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.
Kumachaman, SuperGamer23, Xx1Sans1xx, Legion Riven | RIPPER, juanrojas36, ZONT ADI, VcTeF, gilmar_dubby, mori817221, kabamaru, -CG- BrandonDep, Rujuk, noriokun4649, pwo999, zxc123456789, netgame8534, Aミスティア, asai_kmh, asai_kmh, im so hungry, simasyou1114, kaito-hk, kaito-hk, kaito-hk, lbbjkl, baruburu, liviotdf, Ƀɑɗɔȟɚᵽ, you.mine, towhtaku, UMA1231, Luxrary@Japan, guren5592, tenpura82, sumomo, fjirin313, Gixmock, makino181, inspectherfuzz, Two b0xes, Noah_SW, doratyan65, SpoopyLegs, Extremskilllzz893, Fjjm, [GEGC]Darth Moul, Mistour, Lovely Jyeon :), minecraftt