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.
Satael, zennat.3195, D/INT3, Alversion, kingofgameslaidaoui, TonyCoP_Y, Dalibor, joan-caturro, mohammadstar202, WaterBurnsHobo, Spriggity, domixd266, Tommmike, 9FIFTY, jvp, Blackeye238, Coldfur, BeasTMoDeCaL, daavidcraft.mc2012, heckkkkk