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.
suarezruben466, ballesteroaxel173, GLaDOS668, Mr.WoofWoof, Bomba, ___/\_/º\(Foxy)/º\_/\___, gamerofkill, Dragon_Shadow, tdayonsol12, mohammed.456789, Cross, konto.owsianka, JJ, 2arx, T o T, lovemyravei98, efkantemel6, Batman, bradhinrichs21, LuaFix, Xak-tsaroth, ♥FinnTheCuteUmbreon♥