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.
Max, NaxoKiller666, Exile, Darklin_Spikez, Untitled-2.png, jeffthekiller96, aprogamer5, .;>|<;., HookersAndBlow, Ultimatum, Antagonist DC, Ruby BlitzKrieg T.R.I.C.kMaster, vic, dirtpeanut, kkelloggx4, =PP= 419, [BB2] Faith&Fear (RUN), Wallpox, A.Loopa, That's Just Dandy, caron.paul, (PPI/ParaUnit)169PD, andrewdbuilder, hdtv480p, [EX] ZiM ☭