Am I right in saying though that GSX do not support any Jetway control leaving this to just those dictated by default or the third party developer?
Yes, that's correct. GSX doesn't do anything to jetways, it only *checks* if they are present in the scenery, so it won't create front stairs on a parking that has a jetway. This feature can be user-customizable so, for those sceneries that don't have proper FSX jetways, it's fairly easy to instruct GSX to know they are there, so it won't create un-needed stairs for example.
If so this is a bit of problem at the Aerosoft Mega Airports for instance that seem to depend on AES for Jetway movement.
That's surely a problem (or a missing feature ) those sceneries have.
If there is no direct Jetway control in GSX why is that and will/can that be corrected in the future?
Yes, it will be probably addressed in future GSX versions, but we are not planning to do it for sceneries other than our own. In the long run, it will be scenery developers that will realize that having proper FSX jetways, now that GSX is out and very well established on the market, it's a mandatory feature for a payware scenery.