A couple more Buick web sites:
Buick Gran Sport Page
Stage 1 Registry
According to a book I have the GSX option, for 71 and 72, was only available for hardtops. It has a copy of a dealer bulletin which states the GSX "...is only available on GS model 3437. This is due to different dimensions in the rear quarters and fenders. The Model 3467 (convertible) does not allow sufficient rear spoiler clearance"
Andy