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.
SCOUTSS TF2, Barty2890, Pink Guy, Rabbit[Coco][Azog], Sl3nD3RiLlUm1b0nN13c4sK3T, guisselsa, Amanda.kirkaldy, Krulik CSGO.BEST, Eugene Claude Van Damme25, Xx_Hunter_xX, BaccaBeast | #Pootassium, TheRandomKiyru |AI|, *M.O.T.A* WolfBlood L'irlandais, kristiancarlf.mackay, Biahwtrooper, Biahwtrooper, BingoRingo9000, Shadow | csgoskinbin.com, Thomas, 13720051383, Raferter, cyrusalim31, furdok72