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, re15revisited, Speedy, Speedy, Azeron // pcmasterrace, Gibuya, zegooddoctor, Alianorx, Delta7x, Russia Greatest Love Machine, xB34ST_M0D3_4CTIV4TEx, ThePedoTree, Cold, Zicke Kamikaze, VooDoo, Der Alex, ๖Ͼᴛ Ferox, Peter