You cant change that. GSX is made for bigger airplanes right now. The GE airplanes, the passengers would have to duck to fit in. But that is not possible technicaly at the moment, because it would need a floor and a ceiling and the passengers crouching automatically between those two. So the only way is to live with the passengers sticking out or not use the passengers on small airplanes (or to adapt the height of the stairs so the feet look out at the bottom instead of the head on the top, but that is not much better.
FSDT once said, they plan to release a GE edition of GSX, but that could be years away.