Looks like the Generator Battery Control Module plug.
I'm not gonna act like I know exactly what it does and explain, so here's the GM description:
Starter/Generator Control Module
The starter generator control module (SGCM) controls the flow of torque/energy into and out of the starter/generator. Overall, the SGCM controls the starter generator's engine cranking, torque control, speed control and torque smoothing/active damping functions. The SGCM also controls the four types of power:
• Perform bi-directional 36-volt DC to 12-volt DC electrical energy conversion between the 36-volt DC bus and the 12-volt DC bus for system energy management.
• Perform bi-directional DC/AC electrical energy conversion between the 36-volt DC bus and the 3-phase asynchronous machine
• Provide 120-volt 60Hz AC energy for on and off board electrical equipment.
• 28-volt AC for the starting function.
In the basic, 3-phase inversion/conversion process, 42-volt DC is converted to AC for starting, and, conversely, AC is converted to 42-volt DC for recharging. In addition, 14-volt power is converted to 42-volt for jump-starting, 42-volt power is converted to 14-volt for the battery charging function, and 42-volt power is converted to 120 volts AC for powering the auxiliary power outlets.
The SGCM has two discrete crank signal circuits inputs to calculate engine position. One is hard wired and one is a signal on the GMLAN bus. This acts as a back-up system in the event that one signal is interrupted the engine will be able to start and run although a DTC will set.
