Is the firmware for the A/C stored within the gimbal unit, or is it stored in the ESC?  

It is stored on the main board inside the aircraft, the ESCs do not have their own separate firmware, and certain code is stored in the gimbal unit as well.

I see. So replacing a damaged gimbal unit will not affect the A/C firmware?  I thought since the bin files were being read from the gimbal unit, it was stored there as well