I’m sorry to hear that you are experiencing a belt squeal with your GS when it is started with the A/C on. Though you are looking for feedback from the forum community, were you able to get in touch with your dealership regarding this concern? I would be more than happy to look into this concern further and get in touch with them on your behalf. If interested, please feel free to send me a private message including your VIN, mileage, full contact info and the name of your preferred local dealer. Hope to hear from you soon!
I just had the same issue with my 2011 Buick Regal and it was the tensioner and belt. My car would not make the noise while parked but the minute i put it in drive and give it gas, it squealed very loud. It was fixed and I have not had the problem again.