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.
animerychlicki0, gamerguy8973, sumo25471234, dogukan_aksoy222, [B.A]BLITZKRIEG, josephbante338, Warhammer, Deaf Kikbow, ♤Isaac Clarke, little guy, StupidKey, DrunkenPandaBear™, ۩۞۩ FerMod ۩۞۩, naver, koosh putin(SLO), 감귤 작작먹어, Vexcernite