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.
アレクセイ, The Whitest of White Men, V.I.R.U.S., erolwolf072, olmez.56865, Hoàng, Say!!! Hi, Zack, famelnw, kcmukabi, Nauser, Gamer Person, tai38720, krejwen95, mossesx, usman.edelkhanov43, G2 Rizzo, i.r.vanade345, МореИзБалтики, conly, rudolfhessss, L490 是水我的朋友, ewadan255, I AM SPY, 粉紅怪盜, TommygunJ, TommygunJ, vladdengyd, vladdengyd, mimszix3, Noчnaя БАБОЧКА, pinojimenez07, Somoslosjo, Sam[TIA||RHC], LeHawaw62, Stefan, Murder Inc., rladmlgus00, ☠Midnightstorm☠, ☠Rocky☠, bedirhan_seren59, danielbravoone, jhgpace, Nep (ﾉ◕ヮ◕)ﾉ*:･ﾟ✧, NekoButler, SuaMãe, chrish5168, 额, 额, iam_mr.bin_97 +100 others
