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.
Pirangunter21, Royal, re15revisited, Squeeze, , , Big Çurpraisi, koossa12344, Смысл Жизни, ChalkSauce, rmn1981, *thekillergamer* CS:GO, Cannibal Cat, Cannibal Cat, akottok, ТЫ ПЕТУХ, jorgenieblas, wannes.vanlaer, Mr.Yisus ®™, Sir Scumbag, Pianotugboat, -CSGODiceGame.com- Amazing_, Blanc, [PW-Fr] Gold, The Sovngarde, meowingtons, (TR)Bozkurt, (TR)Bozkurt, Narnio, Crankshaft 5™, Superspy6, doctorwho1227, Timdu45, .::S-RaWeN::., SCP-173, Ruby BlitzKrieg T.R.I.C.kMaster, pi, Warpath-the-drone swe, Warpath-the-drone swe, MediEvil, Bacon, Mr Energized, Transient Princess, A Trusted Friend In Science™, The Unlucky Medic, Frozz, andrewdbuilder, andrewdbuilder, [EX] ZiM ☭