The only real difference now is that GMC has some option package differences than Chevrolet and a couple higher trim levels. Other than that NO difference. Built on the same production lines and most parts that are different are only cosmetic. A few years ago I know that they really didn't watch what was going on very well. A friend of mine ordered a new Chevy truck, spec'd it out and when it came in he was real happy. Went down to pick it up and just about started crying due to laughing so hard. The dealer actually got ticked but Bob decided to keep it the way it came. What was strange you ask?
Chevrolet Dealer- yep, Ordered and built and sold as Chevrolet Truck- yep, Exterior trim Chevrolet Silverado package- yep, Interior trim package Silverado- NOPE Had GMC badge on the steering wheel, on the right dash trim was GMC and ONE door panel had a GMC badge. OOPS guess it was a Friday build....