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.
Harvey, Miss Nirny, dima.averyanov.2019, mossesx, ☠Ļửţţõ☠, bflmn, Миротворец, Däckers, enrikerdu62, Fwiffo™, dodgymike, l.maire c, Herbert Grönemeyer, Wolfman19682005, boblealeonard, Nick Hertz, Lord Megamix