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.
Drai, Addicted to demos, rmn1981, futo999, ◤.SẃαŦ.◥, SpringTea, ТЫ ПЕТУХ, GMgregster, josh_da_ninja, Blanc, brentm.osorio, [PW-Fr] Gold, Abe Froman, The Sovngarde, meowingtons, BigWAT, Timdu45, [BE] JustSayBrent, Psycho '99, pi, Zaku II (laggy addon collector), ACR_Hutch, Mr.Lazarus, Arctic0ne, B.R.A.K.U.M.B.A 2017, B.R.A.K.U.M.B.A 2017, [BE] ViiruZz34, [BE] ViiruZz34, Puke Hammer, LazerSpy, PeculiarPsychopath