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.
osuruk-kokulubok, Tails19935, wwwww369, wwwww369, re15revisited, rmn1981, merritt.matthewj, Hellsvander, Ex-President, Sparki, Kans, Thalia McKenzie, sinius, Warpath-the-drone swe, Warpath-the-drone swe, Warpath-the-drone swe, Ignis, Rawr, Roeder, squeeze, KALTAG, LazerSpy, Soviet Comrade, zdishmant, Braxen