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.
, the gunslinger, applepielord333, marcel.wagner.1, febalbino2015, jumbus larj, cont.maldonado_matias, Big Fat Meanie, <Wakaba>, [ISIS] Jimmy Sav, Riha.Jarous, Shadow, omy829, DK쿠키, Nepgear ♥|Purple Sister|♥, STCG, 【F.V.S】Lord Magorius, koosh putin(SLO), Keren1altman, yann.devos