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.
Faker, SeKrEt, Meeps pvpro.com, JaozinhoBR, BrunoGamer_ BRz, mossesx, Lemonz, GruffyWTF, Video9ames12, Silveron Play, martihernandezmartinez, EthanTee1701, (New Profile) DodderyMedusa89ALT, Uncle Jon, Haydo0467, Team Frost, [VIVENS]AIM.EXE, Baba, Cochack11822, GamingPlusPlus, pgyunus11, Phantom Dancer, Jandre313, wagner-ju123, marcos-paulo11, mr.lol2269, ZiON, Jackcrazy20, j., JusquaLao, aryamansingha, mariano12, Helios#Duck, lincolnspencer13, oscarecruz629, stormcaiminecraft, TheKillerGamer99, PhilR159, frankrositan, sansarr, ShadowDrake85, logon, ArmagdoGaming, Belal 99, guillerbarreda, tuna_kaluc012, legitsnipingg, Xvraiden, RhinoSandwitch, Marksman +50 others