The Game Gear wasn't nearly as popular as the Game Boy, but it was still a cool little handheld system. If you have one sitting around needing a new use, Adafruit has put together a guide for embedding a Raspberry Pi into it so it can emulate all the classic systems.
The process here is pretty tough, and you'll need some basic soldering skills to get the job done. That said, once you're done, you'll be able to use the Game Gear's original buttons to play whatever you want and you'll have a handy little portable system. Head over to Adafruit for the full guide.
Raspberry Gear [Adafruit]