GMC Hybrid 2007 Owner's Manual page 42

If the coolant inside the SGCM coolant surge tank is
boiling, do not do anything else until it cools down.
42
The coolant level should be at or above the FULL
COLD mark with the vehicle parked on a level
surface. If it is not, you may have a leak at
the SGCM heat transfer core, SGCM pressure
cap, SGCM auxiliary radiator hoses, SGCM
cooling pump or somewhere else in the SGCM
cooling system.
Notice: Running the engine when there is a
leak in the hybrid cooling system can
cause the hybrid cooling system to lose all
coolant and can damage the system. Get any
leak fixed before you drive the vehicle or
run the engine.