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.
gianbasilio.nisperos, sebasmoutt, vandroidt1jplus503, boniebest10, dimitrijevuckovic026, RZG_rex64, shin'ah, The Pain, Samu3l d Luke, lertsri, Mr.JooJ, Alphaceph234, GTSMRLoquendo, scorpion18, Unknown Nomad, BREAL$, komavelimusik, hexer54539, hexer54539, com.antilles, joaquinveroon09, Doomfist7576, hamzaamir7910, ColderOrange, OsviLokuo724, pedrohenriquenatividade09, shibashuster058, (THE WOLVES)CYKA BLAYAT, Art Art, krzysztofbalina008, играб-и ГРАБЛИ, djordjesmiljanic143, Bl1tz-Kr1eg, yuriblyat45, Lockensen, mattricci7, mattricci7, WilliamTheGamer101, DyLAN, famlovato, xoraz, Emperor Epitaph, Ice, Tyrannotitan Fan [PL], Pratz777, DemoPan, Sydalo, kaikejcd6, zigortfurkan, airapples +149 others
