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.
Windseeker, [PL] z є σ и, bibrkacity, andriskopoprdisko1, nurlybek, wolfieczsk, bob, vBDSK, Player, (VGW)®Phoenix®*NL*, [email protected], nevik609, :DDD, Dr Whooves, ӉɛɑɗħʋɳτɛⱤ*☠➦, Minimole the Lorax, lonely19682005, KERRY, MagicCat, ERROR, Kresinite, jQueary, Gnoyek, LazerSpy, LazerSpy