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.
nathansullivan1246, DarkPizza :v, Brine02, Sebz the hedgehog 3, sunfeling, kubajunek, spungly, suburban proletariat, suburban proletariat, kaikejch, NZ Vulscar, mossesx, Tumble (D5), [APC] G Froman, alexshulski, Gmanmik, OmnivionTV, rodrirossogb, re15revisited, re15revisited, Mermit, Mermit, alessandro02c, Lance, , BigBootyLuna, honestroleplayer11, DEV_nap❤, [BB] ZoZo, GORYGIRL1996, desinteresado124, Just a Generic Hero, LittleRAT, ///Nyma///, Optimist, Hase, rafa0211.rafa, ☛ApeToe, 2D, devanandma, commie, Lady Serpent, Tango, OutCry, JeffreyWoods28, JeffreyWoods28, Biahwtrooper, Biahwtrooper, deeginator2, That One Atlas +22 others