It's not made in Gm, but another language. Just search it in google, 'Halo Zero'. It's soo popular, and has a full campaign and multiplayer. It was based on the exact story of halo 1 (pretty much just an exact 2d clone) except it didn't feature many weapons. They're currently making Halo Zero 2, which will be the clone of halo 2, only in 2D. You can download it, it's only 20mb, check it out.
Scotty