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.
=CCG= PBeezie80, NASA, MitchBruh, Vape Nation (\//\), [TTOE] Cunt punt, [PR:RP] Manny, Johnson, alecvemmilee24, Zloy Gus, Gigi The Kitty Boss, Ricards3, RubenLittle, Gabe127, [DPA] Samsterminator, Amine, Camo™, -vG- Miniman0404, mark_xero_04, RileeZ, Valentino Joan, Phosphene, Rive, MG | Papa Yeke, Jimmy, adwokatPL, Nuf', Eman0039, Sam, Chris, JustJosh, The Google Man, Mordred, [MVG] Sammy