It's factory installed. There was another Buick memo saying dealer installed AC was not available due to the complexities of the new GS. I'll have to check to see if it was only for a limited time, or the whole production year.
The Baltimore plant put very little option information on the cowl data plate. There would be a '4S' on there for the GS option, but little else. Fremont didn't show any option codes, but Flint and Kansas city showed many codes.
There is usually a body build sheet stuffed into the springs of the rear seat cushion, take a look!