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.
soocool11, mossesx, Phonrage, adlerdani01, YGH1520, Family_wms, laughmaster360pbb, rfswingr, Paint, Pilgrim, Redxoon, o u t l a w, meunomeeberg, Detective MAGNUM, Babadaeus, Squeeze, Leman Russ, The king of cinders, kriegtch, Sir picdino, [OtSSM] Otto Reiter, [OtSSM] Otto Reiter, [OtSSM] Otto Reiter, Ash The Juice Knight, ysraelmoreno, Alexander Preda, Ventuhr (Ashen), Gretto, DECOY panda1, Jeremyzombikill, Darth Vaper, i wanna be like mike, Krobus, Shadow, Friendly Floyd, I Need More Coffee, robbed by a chicken, Pantherlily, and0102pad, and0102pad, ShadowGhostTR, Ichor, Species, Do Names Matter, UncleSHREDDER, UncleSHREDDER, Slottin' Floppies, |S.T.A.R.S| XTerminator, Method Man, georgygv +39 others