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.
Crin, Ethan23rox, Super Courier, Tewo | FutureRP.co, Heisenberg, MR.Mango.dk/DSF, IgNaRIuS, Mystimate | trade.tf, Valkorian, rextheslime1, DickButt | kickback.com, CheeseZ, troy.mallett, Maj, erikvallejo, b.talkingparrot | Gamdom.com", RugZ, bread 1frede, Ilyass|SkinHub.com, SLD R《▪Slandersop_LV▪》, Bopper, Biała Mewa, abdelsalem.deytb, Bravo2YTpl, gamergame067, Panda, John Doe, MR.harambe, SkullBreaker919, Mop, I Vape antifreeze, wikili202, R4z4, Tanong8.3, Rose, Dyna_MiTe`(Trades RL Items), linuxser1234, GunnerCWB | Hyplex.co, BooYahGaming, Totally Not A Scammer, AZZAR, ๖ۣۜDu$tin, ✪[TNK] Panda, tony.debard, The Sloth God, Teddy, AMEER, SynthaSiX, Jeff, Corzza +129 others