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.
re15revisited, Смысл Жизни, :3, ТЫ ПЕТУХ, Blanc, [PW-Fr] Gold, meowingtons, (TR)Bozkurt, Timdu45, Captain Kurtis, Pittapat56, Forg(R)otten, pi, Arctic0ne, Arctic0ne, Phoenix Connexion, Puke Hammer, Frozz