The first person to convert Sega Game Gear roms over to Sega Master System format was Chris Covell. He did like a half dozen games, nearly 10 years ago. Conversion involves a number of things, particularly remapping of buttons, and palettes.
After so, so long, earlier this year business picked up big time! The SMS-power forum has been the hub of it. A few different people have been converting a number of Game Gear games. Some are still WIP. Most of them are done like Chris, where the Start button is mapped to SMS button 1. However, some have Start mapped to down on D-pad on controller 2. They also involved resizing of the game's resolution to fill the SMS resolution. Which is very nice.
The roms are not on the site, only patch files so you have to do that part yourself.
SMS Power Hack Page
Forum discussion thread