Also, having worked in a GM service department, one of the best things to do is to keep your cool and keep your composure. Document, document, document also. Being able to listen to what they're (the service writer, manager, or even technician), telling you, will also maintain an open line of discussion for you with them as to possibly resolving the problem.
The minute you fly off the hook, start telling us what a P.O.S. the vehicle is, and how much you spent on it, and that you'll never buy another GM vehicle or buy from us again, or whatever tyrade you go into, we shut our ears off and we go into defense mode. That is typical human nature. Once that happens, good luck! Calling the number posted above, usually will only help if GM is gauranteeing that they will foot the bill for whatever repair is in dispute. A dealer is an independent business with an agreement to sell and service GM's products. As with any independent business, if they don't want to serve you, that is their right. Just as much as it is for you to never patronize them again. If you're being denied warranty coverage, there is probably a reason why. Not just because you're being singled out! Chances are, they (the dealership service department), determined that the failure wasn't due to a fault of the part and or workmanship of the product, based on GM's requirements.
To make a long story short, a dealership foots all the costs of warranty (based on GM's requirements for coverage), until GM pays them back. Sometimes after an audit (if there are questionable items or a history of questionable items). The dealership service department can also "goodwill warranty" stuff through GM too. Say a vehicle is just out of the mileage or a month or two out of warranty coverage and a power windoew regulator fails. 9 times out of 10, they'll do it out of good faith and usually GM steps up and backs them.
Point being, whether it's right or wrong, that's how they operate in reality. You attract more flies with honey than you do with vinegar.
I've had customers come in, say "hey, I know the car is at 40k (back when they were covered under 3yr/36k warranty), and this failed on me! I really don't think I should be made to pay for it!" Honesty and niceness would've gotten you covered (i'd have "goodwilled" it). But you come in spewing venom, and I go "lalalalalalalalla"