1: does the gimbal warning appear just before the kinematics is failing? , if so you may need to look into the measures it warns about, i do not recall to have seen gimbal lock warning on the AC joint before. The remedy might be to change the to rotational measure in the AC joint to avoid gimbal lock. (This is only if the kin error happens when you get the gimbal lock warning)
2: Then you would need to change the GH joint definition, currently it is a spherical joint with a special arrangement of the reaction forces, which ensures that the reactions points within the glenoid cavity.