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.
francescofrontuto314, Creep Kyurem, [email protected], TanchoOtaku, 1234sparxlithum, Mullic Rav, chimeraskeleton, chimeraskeleton, KiroFoxyScout, oradwane73, murilo.repolez, Mad Matters, Warpath-the-drone swe, theundergamer, Blaze the Pegasus, anicity12350, Dis Dolphin Guy..