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.
Boostio's Waifu <3, RoyalCat, ctin, MikominMM, Monstacat, Raziel [Ru], Minecraftcreeper333, golddiamond001-COMBINE, Makoleti, InflatedEggo, InflatedEggo, Reapz925, -=웃=-, Zortex, GeorgeJ, borusitisan, SKINMAKER, [LxN] Lance the Bunny, dhgfgfg6, -XV[HWEMC]hoyo(away), Xylene, Emma_Rosalie, olkyt, One Certain Mystery Man, Rayts5, Kevmike, Lord Of The Sluggies, rare pep, Susliks**, Thecityburns, Micah, Pancakeattack!, [TKZ] bellum128, kharm-jay, VL4DST3R, VL4DST3R, Chompyyy, asdf, pacman1866 <3, Elite, Wood > projectzecon.uk, (Spooky) Troll, N.O.V.A, cud123, Hds46