This download looks like a save. You need to place it in your saves 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.
\/endetta, MaXXL, francogallardo2006, DJDragonFree#RIPCOD, RHY FMD ☣┼Sgt. Spooks┼☣, nakkalokesh966, koossa12344, Raven, AGRESSOR, toddortiz83, XANTHAR, m3gu$7a31p4ya$0 >:D de Astora, monitorkop4e, Heckelex, HELL CAT :3, deeginator2, ☭ КАТЮША ☭, Red Moon, Red Moon, WoLfIe<33, King Shringus MXIV The Great, nongteam_seka, Murder Is Pink, SuBDivisions23, Jeffrey, Jeffrey, |-KPH-|, FeatherstoneRF
