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.
lena-konovalova91, AironHerder, ZKA, Ofeldw Mikael Artzohm, aaronjr.2005, Bux, azrildyu, sectorbloon, senayserkol47, aaronjr.2005, aaronjr.2005, Vladimir Lenin, theqemists, theqemists, filip.micyk, Cr0x, ThisIsALie, [IG] Tritler, coopermaguire1945, Fakeguy18, mossesx, PimpBlimp5000, Jarod, my.little.pony.and.dinosaur, Steph fuckin Curry, Хер Маёр, thomasrodriguezcabj, [APC] G Froman, [APC] G Froman, vissengood, John Mullins, lidia_palacios15, alexshulski, Sam the Man, Kaan Usta, De Crasher, Kεlroζ | cs.money, J.Raul, burnouth03, dmv28111990, [HP]~D)I(IN™, _f., Ignored By Society, uhnoo, zdravkocausevski, zdravkocausevski, wwwww369, DoggoficX90 - DAT BOI, tommymccarter.uh, werin990 +136 others
