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.