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.
lucassssarceee, endorsedocean63, sasabazilak1111, cavalomito123, matimor35621, GuisoDeNenes, Слышу ZOV, arez5331, stephenwatson2022, Condition Zero, $omeone, PixelsBR, FANTA_CRACK_GAMING, ghost, Wave, dyzanb5, pedrogabrielwms, marcjosefrabina, SMGDF2, maxlayer, AshuroGamerYTgamehag.com, jeralpejose231, Я из России, qjaraemilio, farisaryan2011, farisaryan2011, paz.urra.221, Nashe, d3d3muh4dr, Tito0, migo, mohabmohamed20077, nikoladmitrovic52, nekmakarov, 3020994585, The_One, basmmohy74, emikoliwier, lucashoyosarce, damianpastor1, jesusandresgonzaleslcheyene, pacon778, GodlySprint, DLLfile, kaylloncunha6, mortenalexchan, Dado 1234xd, Zipzon, nadson.trabuco, Skim But turkish +877 others