This download looks like a lua file. You need to place it in your lua folder inside the Garry's Mod 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.
NOVICE_WANG, Pumper, Glitter, Heist, Anglakey, TOO-BEE!!, Magic Shmagic, labrats45, PigeonHawk, Someone, Nic' *Logitech, v.pfahl, ELITE COMBINEXD, [UG] Spice, Direxe, aidanspalding1, Wishmaster, ·٠•●ProF!K●•٠·˙, SHOOTABIRDIE™, ✠ Shark_vil ㋛, [HLWS]Fr4nkM - Just Hoppin', [LFG] SaintLuther, E.ver, Ebrithil Sirdog, Official ² | Nitrogene, Official ² | Nitrogene, OlDGuY, Nuker, [LulzP]Denz, Redline, Natolikin, II_PYRO_II, , yao6killer7, |GR| BadCompany, Centric