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.
The One Ferment, mossesx, re15revisited, Dawson, Doktor haus, Doktor haus, The Chef, ☪Azerkan67☪, sinius, Ω² Big Smoke - kenneth, Babakool, enrico_russ, heRn, TheBlackShadowOfGod, Trumpet Player, Will | KryptonNetworks, LazerSpy, LazerSpy, andrewdbuilder