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.
morganbrown2, [PR:RP] Manny, Johnson, alecvemmilee24, Zloy Gus, Kitty, Ricards3, RubenLittle, Gabe127, [DPA] Samsterminator, [FR]...Amine, Camo™, -vG- Miniman0404, mark_xero_04, RileeZ, Valentino Joan, Phosphene, Rive, MG | Papa Yeke, -ExRP-Jimmy, adwokatPL, [DG]Eman0039, Sam, JustJosh, ~FUŘY~, Mordred, [MVG] Sammy