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.
Jalil_WFT, rmarcell124, Zolaf, Sholankenbob, Jnco85, Necromantic Paladin, SueLiz19, SueLiz19, Prince Nyx, Agentlou44, Clem, timothywegma, Relyt_Johnson, dwayne-theeten, SexyButterStick, duvantoloza925, pivnyov08, darionhampton, darionhampton, darionhampton, boyaltanvir, ❤Latios The Pegasus❤, aman360#pgnn, RcBoss, bellito-bryan, bellito-bryan, gonzalochalo1212939412883434, pvt1175, Sesu 'Firamee, jg9766482, Felix.Sangheli, papitazgamezhd, KapItalist Ball, MasterChiefSon, Macch_Toric*98, [[Mek]]SEBAS&YEYO, dasher132, GPS Paul, Shootologist, Alien, Jamie, TravisBr10, fishy, YDSAT, palito_thomas, real_deafcon118, projectJ.k837, jonathandavila806, 「Generic XBOX Profile」, kingofcats95 +80 others