hmm. I dont have a game gear anymore, to take apart to see how much room there is in it without adding to the back of it. but you could cut down a genesis controller and wire it up inside with a switch on the ground line, would be nice to have a detachable cord. but it would only work as a game gear wile on, and a genesis controller while off, but would not work as a 2nd screen to see the genesis on. unless someone knows of something different.