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.
kevinwaters1976, blakeblackwell463, mossesx, Maxou, GreenFox, Asdram {FR}, [M]ythical, leonardodente_, Fabian, ferdynand kiepski, isabellesaguey, картошка, Johenzo, 406House, ╰☆╮ sarah ╰☆╮, ╰☆╮ sarah ╰☆╮, BDA, Nalyd, [NOR] Mitch Musicman
