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.
Manning, [♥] Ale, DiddyFanGirl92, Sora, ArmyLegoMan97, Didgit.Sergey, kaoru124, airplane02, We Shall Go Together, Xeilung, KHò-ààAHH !!, jeffthekiller96, Biahwtrooper, Biahwtrooper, Eagle♠Eye117©, ¯\_(xay)_/¯, ¯\_(xay)_/¯, Riha.Jarous, OmegaMario89, Bruno Zillin, remarpc, ToastMuffin, BOT_09, hjmen32, sinka1031, sinka1031