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.
All_Time_Gamer_3R3, vicente2599, JP420, kugibg, re15revisited, re15revisited, heatsaber, parkgw0701, zinkia, tjpetpetty2334, ilya.mikhalev.o5, Noodle, Bread, bobperez73, punchedfever8, wwwddfafff.aa, Ack Ack, Pedro_leite_06, emanuele_fusaro, FreddyTheGamer, Prologik, munroedominic218, Biahwtrooper, [HDG]2Sneaky, ric2017 (G.E.W.P), butter, delicious butter, xX_THE-MINECRAFT-LORD_Xx, milan_luca, dylthomas.19, Gamfu64, Dusty, jguiser98870, "Omerta RP" Kospé, Sans, CaptPatrick01, NightGlow, ramisik09, Jeff Dunham; Relevant Comedian, Tamo, Winter, [L²:RP] Mustang, BLaMotte, NCFD43, hamzah710, Nidolai, Nidolai, awesomestuff26, [BA] Brucektrain (DEAF NO SOUND), [BA] Brucektrain (DEAF NO SOUND), crazyboutrocko +8 others