If you load up a Game Gear rom on your Genesis, sometimes you might get a small screen or two from the beginning of the game with false colors. I haven't found one that is actually playable yet. A proper Game Gear to Genesis adapter would work similar as the Super GameBoy for SNES does... have a Game Gear built in to the cart. But it doesn't end there, oh no! Since the Game Gear has FAR more colors than the Genesis and the TurboGrafx (it destroys these weak-ass systems) it'd have to have it's own video out.
I really don't get that the Game Gear had so many colors. All of the games looked far worse than Genesis games. Most didn't look any better than SMS games at all. They wasted all of those colors 99% of the time, I think. What a waste of a system which also has the world's shittiest screen. It deserved to flop and I'm glad it did.