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.
Barry, ThereIsNoSteak01, Snoozy The Pirate, FirstOrder_001, Bartey, The Melon King!, The Melon King!, BenjaminA12345678, FlikerZ, guilhermezommaneiro, MetroidJunkie, chaconrho, DTGZombieG88, Mr ?, Fungorum, Sir.Temmie, Sir.Temmie, combine2005, ekporn, ___Крош___, Flowey, Kairamen, Ghost Pancakes, nerd535, themackboy105