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.
Player, Shuri ♥, extrasven, Val, kgergo5800, SucoTashi, Zi, Ruby Soho, nanashi86, re15revisited, joun_lord, Смысл Жизни, rmn1981, :3, ТЫ ПЕТУХ, Thermonuke, s1l3ntwÖlf59, josh_da_ninja, Pianotugboat, Blanc, [PW-Fr] Gold, The Sovngarde, meowingtons, (TR)Bozkurt, BigWAT, Bao, The Wolf Duc Volpe, VenomBlizzard, Forg(R)otten, Kelevra, Plum, Arctic0ne, Arctic0ne, [BE] ViiruZz34, [BE] ViiruZz34, Hey Kid, Wanna buy some Peace?, , Puke Hammer, Frozz, Soldier, andrewdbuilder