GMC Troubleshooting Questions? Ask a Mechanic for Answers
Try pulling out on the trim panel while loosening the nut. This may hold enough tension on the stud to keep it from turning. This may require a second person to do successfully.
Now that it is apart you can separate the nuts from the studs. Clean the studs and plastic trim and use a 2 part epoxy like JB Weld to reattach the studs to the trim for reassembly.