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.
Piloh Jolie-Schitt, Detective MAGNUM, Max, NaxoKiller666, Exile, Darklin_Spikez, 1-800-273-TALK, jeffthekiller96, aprogamer5, .;>|<;., HookersAndBlow, Ultimatum, Antagonist DC, Psycho '99, vic, dirtpeanut, kkelloggx4, =PP= 419, [BB2] Faith, Wallpox, A.Loopa, Seductive Spidey, caron.paul, (PPI/ParaUnit)169PD, andrewdbuilder, hdtv480p, [EX] ZiM ☭