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.
alienmegamatheusbombs, Alias_of_Grey, mossesx, ᕕ( ᐛ )ᕗ TheOnlySalvation, gamercouple81, thelegendofzyro, bobacrome, manaphoenix, choochit.plessis, burtlancaster1113, wasker12391, Wolli play, kobolotin, minecrafterlink, Hungry Pumkin ( ͡° ͜ʖ ͡°), Wolfman19682005