Does one need to make cfg files for each airport, in order for instance to use the marshaller, pushback, passenger boarding, even if other aspects such as catering and baggage handling would not work?
No configuration is needed for GSX to work on any random airport, that's the most important GSX feature: it works automatically everywhere, the "supported airport" concept doesn't exist.
However, you CAN customize a scenery, to improve and fine-tune eventual specific issue with a parking, like buildings too close to the stand, static scenery objects in conflict with the vehicles default starting positions, parking radiuses, pushback directions, etc.
Also, is there an updated consolidated list of aircraft supported by GSX?
When we started the list, GSX didn't had the ability for users to easily create/edit their own airplane configuration, so an updated list of airplane supported by GSX was needed. Not so much with the current version, because creating a new config for an airplane, especially if it doesn't use non-standard variables for doors, is very simple, shouldn't take more than 10-15 minutes in the graphical editor. And the most popular payware airplanes that usually require the more difficult configurations, are all supported.