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.
Addicted to demos, rmn1981, futo999, ☆.Տen乙Θ.☆, SpringTea, ТЫ ПЕТУХ, GMgregster, josh_da_ninja, Blanc, brentm.osorio, [PW-Fr] Gold, Abe Froman, The Sovngarde, meowingtons, BigWAT, Timdu45, [BE] JustSayBrent, Ruby BlitzKrieg T.R.I.C.kMaster, pi, Zaku II (laggy addon collector), ACR_Hutch, Mr.Lazarus, Xper, Xper, [BE] ViiruZz34, [BE] ViiruZz34, Puke Hammer, LazerSpy, PeculiarReality(Sick)