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.
thuan_cccccp, Tsubaaki, Kaz, Cat, kkosio, MegaClever ฉลาดสุดๆ, IksDe #I_suck_xD, Kuro Naichi, shadenator, BodyulCG, [NL]Vi$i0n[NL], Vyey Slak, HiiX #SS-Network.dk, Barack Osama, plax23, Mr. Metric, D2R-Rowdy, Gon Kun, WallStreet, Nekromantic, Mr. Sixx-Seven