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.
Sergoo, mossesx, Some dingus, Some dingus, Danald Dack, Just Gold, swordmanck, 『 』, Se`rg SgriD, HugglesGamerGirl 2-D is Sexy <3, Jerageputaindejeudemerdefdp, Redjfulfer, carlyburd, Lizard Wizard, captainnikolai195, [BGN] Dave, maribeth_munoz, sk8tbdkid, Finland the Man, Festive Shaun, Pikmin, BigWAT, WinningRook, BrianAxel22, Duffy, STAR-LORD, Pixel, Pixel, Green Mario, Mka0207, Dankness Kima, joe.aldred, Puke Hammer