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.
Samurai-Jack, ProfessorOzpinJohnson, Hommie_Panda2134, re15revisited, Con Frunkin, Axogen, freddy.kacper, Something ಠ_ಠ, Saysay111, G-MON, KnzenoHun, Salt Spitoon, justincreator, ayin.mateo, [FS] GoRыNыч, Engineered, The One Freeman, ytp123, Skynamic, babahiano_2005, redstonekjj, Papa Dremek, A Deep Commitment To Konata, Spastik