NHTSA Complaint #11523040 — 2015 GMC SIERRA 2500

I am having a issue with a low milage GMC Sierra. The truck is a 2015 with 24,000 miles. When it was produced the truck had a 100,000 mile warranty but has since lasped in time. The transmission on this truck is a 6l90 which has had several know issues that gm is aware of. This service bulletin is just highlight of the issue gm bulletin 19-na-153. I've talked with several transmission shops and event saw a few posted online about the transmission issue, my issue is gm will not stand behind their product, I have contacted GM directly and gotten no where. During my conversation with the GM reps on the phone they did agree the transmission should not have blown at the milage. My truck is garage kept, doesn't go outside in the rain or snow. I know there is a class action for the Allison transmission and the 8 speed but not the 6 speed that I'm aware of.