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.
IIIIIIIIIIIIIIIIIIIIIIIII, [Wolf], marcusd93johnson, jakubiskerka1, sharahspencer31, Joseph, Eliassilent98, timur123456789098765, GokhanCnK, Harambe Uchiha, graslukasz, nigel92356, TheSexualMushroom, jafet.nimrod, Tim, closeshave, Tephera, Jumpman Jordan, Eradius, selling servers lukefulleylove4, Larry Lead Foot