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.
angeldevon2011, DarkPizza :v, TailsAndSonicFan2005, 5, huekitonhenriquecastro, PeePeePete, leszukmarcel Key-Drop.pl, sla00364, themostlively, carlostabiaxd, pasha.larionov.08, littlesparrow99, ilicg8490, KKiller, zigortfurkan, Rahman, pabloschachner, dyokimura, Mr.Banana, ewicks2004, Drew SlxyZ, ﮎtigma, Balkaptan