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.
memowthehapticbunny, mazedummy, Keepo, LLBLucas, Scott ♥™, Fearlin, TYR3G, judge judy, [Ger]Tom.S.Schiller, Kami-Sama, LouisD1311, Jarkar, Nerolex, UncleSHREDDER, Royal Twist, Emmet, KorruptedSo, Lord_Flydon, Icewine Rose, NEO_EX, Ruby, OGvisualz, O'Nesix, [EX] ZiM ☭