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.
Ghost, Key111220, Pants, [LB] lloebet, My Dudes, Volcom Astro, Yoshi, Gaylord Robinson, Luca94, Daniel, Hades, 蚊子, chronoman15, Marson, TennButay, Wuppertaler93, stefanPRO, Grandma, sukoisu47, sukoisu47, |UGA|PatPeter, -SIP-LordMycroftツ, Emilion, [Anime]Swedish Yolo, [HaG] Killerdog200 ¯\_( ツ)_/¯, Ghost, HAKER, [TVC]Crowley, Gandalf the Ale, Vector, Vector, (DEB) Scares70rm, manguyman, [MAFIA-L] Leonard, 89Mendo89, Psycho, Doge, Mr. red panda, Unicorn, My name is Puzzle, [KOR]joohan03, Rodinavian, [BBL] StahliHD, DONUT, Spurks Donut, 코마코마, 7coil, 123, FuRyX, FuRyX +30 others