That's going to depend on the power requirements. Look at the back of the Gamegear near the power plug. It will say something like "12v, 1.5A" - meaning 12 volts, 1.5 Amps. Then look at the Power Adapter from the Genesis. It will have written on it the power output. If they match then you are good to go. You might need to replace the connector (find them at radio shack). Chances are they aren't the same. I have a box of old cell phone adapters for this very purpose. It's amazing how useful the have become.

n2