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.
ausvinen, Monarch, ☢ Jesterxd, M., Bunneh, Tristwyn, Xeno, Tyrant T100 |Iced|, DellaDellsies The Space Giraffe, tgang, Erja1, Mittenkitten, Kittren, [KOR]혼살, Megazord Prime, Fish, [Hababo] BaDBoYsHD, besttsonev, Harridas, matt black