This download looks like a map. You need to place it in your maps folder inside the Garry's Mod 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.
re15revisited, robinsonht221, Squeeze, guillermov2058, falseprophet141, :3, *thekillergamer* CS:GO, akottok, ТЫ ПЕТУХ, spectral hl2rp?, Frankenstein, decadent_uk100, petersmail, I sell money., bobybai2004, codec459, Sir Scumbag, GimmeSnuSnu, superman-, Techmo, Blanc, TheSkagra2, [PW-Fr] Gold, The Sovngarde, meowingtons, (TR)Bozkurt, (TR)Bozkurt, DjAtomicx/TheRaf3D1, BigWAT, THRAXX, pivotman319 | trade.tf, Vuthas, gloowb, callodutyheroZ, Deputy Slayer, President Nipples, Ruby BlitzKrieg T.R.I.C.kMaster, GJZupaCZ, pi, jaredjb123, Pryde1177514, andreas.limsakul, =PP= 419, Mullins the Great, Bacon, Bush Tucker, Frozz, Sephurd Duckie, Giga-Pepper, ViktorKein +2 others