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.
SLEDGE_HAMMER, s-mukataev, IcedAss, jan.hymr, francisco.costa.morais, mj01032003.mj, bflmn, geoffroy, crowbardz, ibnucandra395, nabilyusof, cubi290306, 743307084, asdwdwad, Romario, jeffthekiller96, Biahwtrooper, cAiLLoU, Riha.Jarous, Agent Orange, Agent Orange, Agent Orange, Agent Orange, Dune Jumper