The Sonic the Hedgehog Game Gear prototype is a demo version of Sonic the Hedgehog for the Sega Game Gear. It was compiled to demonstrate the Game Gear port to the gaming press.
This is a very simple prototype, containing just the first act of Green Hill Zone, which plays on a loop. It shares more in common with the Sega Master System version of the game than the final Game Gear version, having not yet been optimised for the smaller display. Despite this, it is a fully functional, albeit short, version of the game.
Other levels are accessible through hacking, but as the art has been removed for other zones, most crash upon startup.