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.
Pitt #CM, TR1F1N1TY, SUPER TOST, [NG]★Panther★, TheManTheMyth, mzsianmorales, [QG] Crane, [QG] Crane, F2P N00B, TAILS988, Tubbz, [T.A.B] Springtrap, Reaper, Nraaax, Herr Spaztrout, [KGN]KyrusGaming, D.ツ, ĴOŢA, mubasher.shaheen, Magnus, Dr.IrishDan, Darth Vader, ☃❆[SM]RazorNight❆☃, SFAriel, Woky, Mongrel, Clockwork, Neon Umbreon, BOB DYLAN, Zuruuaku, DieAnotherDay aka Mr. Coala aka, James™, Rodinavian, Stealthy Zoroark™, PonyEB, Matt, Strudi, Joy Ferret, Solar, Yohe Chambre, Nidolai, Nidolai, Heyblue, Mr. Moostache, [TG]BestOf, Mr. Brightside, Kolorek