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.
Axis-KT9, re15revisited, DeadCat, varol.engin, CeerTing, pazuzu2010, kobolotin, Exirous, Marciano, MrWARIO (dead), MrWARIO (dead), DJ Bigmac Master, QuasarBarkas, l.maire c, DeesCacho, stabkamay, polllalala, hjmen32, Dune Jumper