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.
josephfaridhuet, chinjung667, wct1962, Piro Winfield, daniel.rowe1995, Grrr76, CaptainRunaway™, bertisis, fpdlittleyoda7, Ruby BlitzKrieg T.R.I.C.kMaster, _.:elmolestoso:._, Warpath-the-drone swe, , Reverend Jesse V92, plastovnik, Crazy Gentleman, msatour51, Gnome of War, Soldier