GMC Troubleshooting Questions? Ask a Mechanic for Answers
It's probably just low on refrigerant charge. 7 years is about right if it has never been recharged. All systems develop small leaks because of the extreme high pressure in the system.
Most any repair shop can recharge it for you and it doesnt have to go to the dealer either. They do make a kit for a DIY'er but is kind of complicated and not as easy as they advertise. If you want to know how to charge it yourself i can give you some instructions though.