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.
[Veteran]Ace of Kings™, Oskar, sahoka, purple smurf lady(I am Plen), MedEvilSourceror, MonkeyKidz, Fat Lobster, dhgfgfg6, [Hamachi™]Jeff The Killer, SHmeluga, SHmeluga, Cre8or, Fasteer, Seris, LagTerrorist, Meow, ๊K๊a๊n๊k๊a๊, [SiN]TobuBiguru, Frozox, Lucky Luck, Lucky Luck, Gulagboy, Wolfie, POLE, Zigzagoon, [MG] AlertIowa1234, A.R.C.A.D.E. JEFF, yomustdie94 [TRADING]