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.
gaming55.teo, rokasrazumovas005, PRO_MasteR, H.Eren Albayrak, ♥♦♣♠Marek♦♣♠♥CZ, He||oween, tindail, jeffy_lorao, manfromarz, Lince Bélico, XiloFox, Xan42, elfrikidecaballito, TF2MlgEngineer #TeamHeavy, Munchala