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.
Anderots, MaddieGrace19, Leap, ronni, re15revisited, ▓▓ ATILA ® ▓▓, Cykamoto, JellyRaven, TheMrShooter, ragexlukii, AsianKid, JrMystery, Weaboo Jones, NickTheGreek, Biahwtrooper, MatildeBravo, exdex1000, DeGroteKarel, Ezüstróka, Turittu., InfinityGreen∞, maruko, player001, chol654, Dune Jumper