First post in a month.
This forum is a real ghost town isn't it?
The SMS Plugin has space on the circuit board for a Game Gear slot. Of course, you'd have to supply the socket and solder it on yourself, but it would work. I could imagine some sort of deluxe version with both slots, but it most likely won't happen right now.
My only idea for getting a GG socket supply is finding Game Gears with broken screens/sound/controls and harvesting the sockets from them. Either that or destroying perfectly good Game Gears, and nobody wants that.