Does GSX have anything at all to do with jetways operating?
GSX doesn't have anything to do with jetways, it's only able to detect if there is one ( assuming it's an FSX animated jetway, not a static one ), in order not to display front stairs and a passenger bus in that case.
This was explained in the GSX manual, Page 11, in the blue boxed paragraph.
At Page 23, you'll find an option in the scenery customization, that will allow you to help GSX detecting jetways on airports with static ones. This of course won't magically make them move (it's up to the scenery developer to support this), but at least you won't see front stairs and the passenger bus on a parking spot that has a static ( = not detectable automatically ) jetway.
FlightBeam's KIAD works but KSFO does not.
That's normal, I believe they added animated jetways starting with KPHX, but next version of KSFO will likely have them too.
EGKK from UK2000 works
Sure, they are smart developers...
None of my FLyTampa airports work.
I believe some of their latest sceneries have *some* animated jetways.
LFMN from Aerosoft does not work.
Understandable...
ALL default airports work.
Of course they do...like all FSDT airports since 2007. I guess you should suggest 3rd party developers still making statinc jetways, they should offer a feature that comes standard with the default scenery.