@mitu

Great, I could disable changing the internal color palette in /opt/retropie/configs/gb/Gambatte/Gambatte.rmp:

input_player1_btn_l = "-1" input_player1_btn_r = "-1"

and put fast forward to the right shoulder button with

input_hold_fast_forward = "5"

I also added

input_rewind_btn = "10"

to map rewind to the Retroflag button from the gpi case 2W and

input_toggle_fast_forward_btn = "3"

to toggle fast forward with SELECT+Y.

@sleve_mcdichael Thank you! Now it's working!