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.
IonutTF2_Spy, mossesx, sorewashington, Skeleton2007, me, K r i s p y, Barry, ThereIsNoSteak01, Snoozy The Pirate, FirstOrder_001, Sniper, i am retarded xd, The Melon King!, The Melon King!, BenjaminA12345678, FlikerZ, guilhermezommaneiro, housdok78, MetroidJunkie, chaconrho, DTGZombieG88, Turducken, Mr.Shuffles, Fungorum, Benjalino1, Sir.Temmie, Sir.Temmie, combine2005, ekporn, ___Крош___, Flowey, Kairamen, Ghost Pancakes, nerd535, themackboy105