Damn, that sucks. I am glad to see the Dealer is standing up for you! That dealership deserves some major props! So rarely do you hear of the dealer stepping up for the customer.:rockin
As for GM, its simple. Can they prove the failure was due to an aftermarket product? If there was none installed, according to law there is noway they can deny the claim. I think a letter from your "Attorney" to this gentleman might help things along. Start some research on the warranty laws and the Magnuson Moss act, which probably won't apply to you as you state the truck had no power adder.
Stick it to them, you deserve to be treated better.