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.
Jacob Rees-Mogg, ♛ Harris Momogoy, Rice Lover, =CCG= PBeezie80, NASA, MitchBruh | Crimson Gaming, Vape Nation (\//\), [TTOE] Cunt punt, Manny | AstraaTv, •Fellix•, Johnson, alecvemmilee24, Zloy Gus, Gigi The Kitty Boss, Ricards3, RubenLittle, Gabe127, [DPA] Samsterminator, No Name, [SIYL] Faety, CaMoKiD✯NiNeTyFiVe, -vG- Miniman0404, mark_xero_04, [GL]Silver4Life, Rilee Czesław, Valentino Joan, Phosphene, Rive, MG | Papa Yeke, Jake T., adwokatPL, Nuf', Eman - OasisRP.com, Sam, Chris, JustJosh, The Google Man, Mordred, Sammy