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.
Spyro, unopar, MLGDogs, richyyc, tpPT #CSGOPoints.com, blackestablish, TazManiia, ozike21, didactaflood, The Dead 95, a1_ahmed20, Shrek, 017135, AceZzHD, eli.michaud, rexetterra, manuanoca185, FortressGaurd2012, Tusk, Pikopaul, mayatr, Heapons, kirbygame126, AlanUsFeu, A.Loopa, Jafar may need glasses, Sokkah Abdelkader