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.
abian_lsd2000, Seismic Alien, sonic, sonic, whosdatsuga, Clammey, Clammey, Noel_RD14, scottydog1989, Eckis, Eckis, saylor, sonicdudeatdawn, jeremywn4, xxMaxGaming500xx, re15revisited, tawseeftahmeed656, donmonic19, bobcho03, Bubblygolf, jafet.nimrod, bunnygamer, shinyhunermaster101, danmccann2003, A641, angeldo, nicdoy, nicdoy, romeowilliams351, romeowilliams351, colby1215, mrfilmstudio, Saysay111, MetalheadMoose, chepemaziso, Bobyian, yanedimitrov612, nasr1355, miguelcasper033, joawnai, jctaylor2025, Theepicguy, jakeboyharris, Dexortis 3.0, BlueTiger, Sr. Habichuela [ESP], King_Bushie, Sir Oswald Mosley