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.
Macch_Toric*98, [[Mek]]SEBAS&YEYO, GPS Paul, ItsOriginal, Jvmie, TravisBr10, fishy, YDSAT, palito_thomas, real_deafcon118, projectJ.k837, jonathandavila806, kingofcats95, dljackson2987, tron1011021, christian071201, Ben, Misfire | IrͭoͪnͤDuke, mathias.cadiou2, cubic_control, robercarrillog, brotime117, suellans, Armstrong, Mr. Traitor (AKA Joker), ghelt2000, omega32b, Marcel, Chris BRUH?, ShaikanWarrior, Xiwai, maribeth_munoz, glennjayxiong, alan_agml, DroidZeroX, Titties, LastRavager, F3N1X Z3RO, xEmortalDeathx, mojache3432, Delta 38, Mr Quasar, elspartan154, ness4725, jjsloniger, Nanook, Rookie ODST, Moat, coolishpizza, BrianaSFM +30 others