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.
andre_jar, WarriorxX2, Funtime Foxy, stefant19901, thunderking00, alashi.mosterskull.monsterskull, cathrinehubbs, augengeradeaus, ColbyErin, BokMustang, Shaspio, Knot A Furry, 0shy, kelmanfidelio, beanybeans2003, Tundra, [PMC] Portable Merc Collector, ttiadavis, Inseptic, lolf5669, ardhocool, A Dire Situation, Smooth Brony Transit Fan, shizugami.ex, i am a cute pig, Gilathiss, The ponyfanuhate, christian.hexum, Kawaii Potato, chmielu, SapphirexGaming, BarneyTEG, blackviper4362, Marasmus, szabolcska2007, jaspercassie, SLEDGE_HAMMER, vielvladimir, Lambda, kristaps6484, Floppy Disk, InFamousKiller37, the dark as shadow, funsy, absolute_k, SovietGuy, semragurbey, RageVolume, IM THAT GUY, chuyito09.jag +158 others