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.
Plebby the Plebapus, LAGKAGE, WWEHDCreator, aHappyTedybear, MurpLee, legameurduweb fonda [oreo rp], Kali Linux, fantome, Cain, mkounkour, JT Machinima, sneedy.izi, SharkGamiing, OverKillHard, slaven2111, Zenza/Synx, Sneox20, benj9029, [TB] Taylor, hugothomas4, mehdi.hajfani, AllahuAkbachar, datad02, [LG:RP] Henk, Plutonium2222, Kaka, Nugaty Sk, SkyGame [cyrillou50], RedHotFoxTrot, Eskil