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.
HorseFighter, *Nervous Finger guns*, Aymenelbaz03, Johnanater, maxbluechip, danielb07.bd, BulbaDwek, StatTrak™ Doggo, Aka Ruu, WarpedTech, Overwatch, Reverse Cowgirl (J.F.K), QG | Bob Dylan, , dan.butkov, HUgeekO, ChewGZ, laurentscharff7, [BIA]bstar9, Headcrab, Main Fighter, Alperen494, The Saw, bigmag, Kill Colonel Kurtz, Aubrey