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.
DnB//^2'^7TrojanHorse^2., stefan.03, nikola1mk blw.tf, Your_Lovell, coachs20003, Sans | trade.tf", NightmareDavid05, Blesk, Di4m Gaming, an99907, serbiatv2, BokMustang, pmarcus581, kgergo5800, [email protected], Una Calavera :v, gamingbunny607, selcukkuscu22, People Pray Peace, Communist Trump, Source SDK TV, Dadak, Parademon on the Right, rhapsodic, whitedark670, Canzandridas, bradobedencio, bradobedencio, fathan_alfajri, trayanotnationtr, 1228256275, joseseb007, ffw79, irongame1014gt, Sauerking, Sauerking, 1Dogg12, imscoobywiththedooby, Steven, trilicle, markigamescz, trece, biocide, CyClone, melvyn080505, Angymine SHREK, drumjedi76, [Fog] Epicfacer, Six target, imddeas +470 others