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.
Lord_Draculea_43, AKINCI, seidrakhimovich, georgijakimov234, delorenzo, Doctor Idooo, L u f f y, Sorry君, skullslater12, cilgintayfa576, carlossantos33270, del.kay1976, tylergarwood7, tylergarwood7, faizallivai, lazareva275, julian2k4, cristherabbit123, RexxlxD55, gladivanokenny4, Gabs, Mr. Troll sonar66, pelleaufeldt, sonic.aguilar45, Doctor kyle, an1tou4 gamehag.com, shibashuster058, josyalencar34, tacoxp123, xdarksiderx1, R²-YGO, Coringa | SDB, Cheryl♡, Shadow, Abelzombie03, RPZ_TD_12, leonelmorinigo67, diablillored, The Real Fat Cartman, shmyglev2002, boba2334, Miu Mellow, Flusho, giankrouz, erolwolf072, alvinminecraft07, dylantrascierra65, ✔ Your Mother [Moat.gg], Mattricci1, Blanco ㍐ +367 others