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.
RED Floyd /(Weber)\, bautista13angelica, CharlieHadronTimeAgent, SOMEONE, Flapjack Joe, frykore, Steadart, The Master of the Showdown, DarwinPlayGames, Shranzkiller, Nage33, petersmail, pstratto, jeffthekiller96, jeffthekiller96, Biahwtrooper, Biahwtrooper, smosh_2556, patrickhu2004, CaptainRunaway™, Soldier, Sneakman, dashiellw812, $∀Я!Ξl, Dr._Hax, Icewine Rose, lonely19682005, jadenford360