GSX will indicate if there's a docking system only it was MADE with GSX, so it will of course work with all our sceneries, since they all use GSX to create their docking systems.
If the scenery has its own docking systems, GSX doesn't have any means to recognize it, since it might be done in lots of different ways depending on the scenery, and there isn't any standard to create them (FSX jetways have at least one official method to be indicated, which GSX recognize), so a way to list them should be different for every 3rd party scenery.
When we'll release the next update to the scenery customization editor, we are planning to allow user to place GSX docking systems in every airport so, GSX will then recognize them, but if the scenery already has them, it would be probably best to use the scenery provided ones.