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.
sergiomagana2013, Aurélien, B1BattleDroidEngineer, KUB!, j2bq20002000, [PN]Sir William, fernando28_07, andresdeleonpaniagua1997, quackbar, JediKnight122, SamuelExceed, oleguerc, mastermodder13, ThisGuyRob, 死体, Mikołajek xD, [FN] Shots, Boost, Sz.Gergő, Dashiell, Kyle Fin, Kraken10, Darkness-/- Yo Boi-, Absolutely Arly, Daniel, Lord TL, Shawn, Pickles Cage, Britboy, cs go sucks lol cod is better, Optiliser, PichotM (Matt)