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.
MIKA, Datasnok, qp, qp, [HOT]Одмен[4CH], Говнях, Green-Green :D, [SRP] Dark_Daccat, SuRayfer, Derpy Hooves, GdNike_Niker, Warline, The Ultimate Garys, WeberZJ, ColdPhantom, ProdokzGaming, The Rocket Gamer