Hi! It’s actually far from impossible. This game is programmed in GBDK, which also runs on the Game Gear. In my opinion, the biggest changes would be to the graphics routines and, of course, the Game Link mode. I don’t know if I’ll be able to do it, though it’s something I’d like to try one day.
In any case, over the next few hours I’ll be publishing the project’s source code on GitHub, so that any skilled programmer can port it to the Game Gear.
I’ll update the game’s page and post another comment to let you know about this.