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.
megias7, Kai, Poroskills, [AGS]BrainyPlayz, DarK-MVD CSGOBubble.com, W4ttou4t, Hyperyop, Harley Quinn, Milchschnitte, imaplantTV, Jet, [RGT] Laskae, friendsxfriends, alex42a, superlogicalgamer, Alucard, Tiemos, zachbrosgaming, GalacticCat, doge, TTown, [eXo] Mr. Exodius, [FT]SigFarina, [FT]SigFarina, MrMidox, zombie11x, SlyDerpy | Trade.tf, CØØKIE MENSTER, Theronguard, Enzyme, Han Solo dies killed by his son, sugar, chainedtick, Mr.Spoops, [Sn!P3]Boub66, [Sn!P3]Boub66, Grozger Zasrogtiveslin, matheuscruz58, Nivlac ( ͡° ͜ʖ ͡°), meli32.1977, Samhamnam, MorbidFeast, Frost, mondelsson[BGC], linkz3, |-NCBA-| Mr.sexness, Rouz, Waffle Boy, superjackp5, Cubic_llama +44 others