Why not just add a TV out and a pad port to a Game Gear? It's still not easy but it is certainly do-able. The pad port is quite easy to do but works better with an MD pad as it has the extra button required for the GG controls. I added an extra button to my SMS console to get round this problem when I did my GG/SMS hybrid console. I may still add a TV connector to it so I can play GG games on the TV as well as SMS games on it. You can see the extra button in this pic, just to the side of the cart slot:
This is a recurrent topic. Unfortunately it is not possible on a vanilla SMS it would require really complex hardware hacking. More easy (relatively speaking) is to hack games on a case by case basis, but losing on color fidelity. It takes long hacking time and good skills to do that thought. See Chris Covell work on this.
Here's some pointers if you want to read more about this topic: