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.
the killer frog hyplex.co, MasterSonshine, kknaulak, TheDarkRealmX, Mr.K, [SPD] Josh, [MojoCity] AL., Sgt.Fast [CSGOPoints.com], Kayxy, Mal, scarbrobryson, LeL, Rire, Pawsative, ZackCrisis, iZeup, Timothé Willow, Johnny, Wanni, Meignifico, Owain Owjo, Jlx7, Andreblue