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.
Selkeer, Netherrealm88, Jayzoh, denny1104, K1PRO | www.topmods.de, peduh parkur, [Ger]Tom.S.Schiller, Chilybones, DONET, Frittilulu, Noxi, MetroVia, ♥DerNikls♥, Michael Möwes, UncleSHREDDER, Gorfin, SNIPSNIPE, MistoCraft99, Rückhandschelle, WhoAmI, sapper man 2, EstoyBolaoCSM, The Renegadist, meharryp