I can see why some people actually thought this might be a new game. Sonic Chaos was only released on the Master System in Europe and South America, and was one of the very last games released for that system.
As a result not many people know that Sonic Chaos was indeed released for the Master System. It's only too bad that Sonic Triple Trouble which came out a year later was Game Gear only since I find that one to be the best of the 8-bit Sonic games. It'd be nice to have a Master System version of that game so it could appear on the Virtual Console as well.
Yeah I know all the Game Gear Sonic games are on Sonic Adventure DX, Collection Plus and Gems, but I prefer the Master System versions. Having a larger screen which the Master System versions have definitely improves the games and in the case of Sonic 2, the Master System version made that game somewhat playable.