In iReal b I made a custom drum mapping.
For example the brush has different sounds on the up and down strokes to better mimic the real sound of drums.GB's drum kits don't have that many sounds so it's not going to be compatible.
I can't give specific mappings because they are different for each style so it would be very time consuming and still would not work.
When I first made iReal b, the tracks weren't meant to be exported to midi, that's a feature I added later and most people use for the piano accompaniments and bass lines.

Massimo Biolcati.