I kept getting tripped up on the question "if a duramax is not a Chevy, what is it?". I kept putting Isuzu. It didn't Don on me till the question changed, that the answer was GMC. Might want to clarify that you don't mean who builds the motor.
I always thought GMC meant Gay Mans Chariot.
I seem to remember a long time ago someone said that Chevy trucks are a GMC with option X88 "Chevrolet Conversion".