But no matter what I do I cannot get rid of the 3rd party jetway.
Maybe this is still not clear:
You can't and you should shouldn't "get rid of the 3rd party jetway", that's not how GSX is supposed to work, the GSX jetways can only be used as replacement for default jetways at default airports, and the Config panel in GSX is used to PREVENT GSX to replace jetways on 3rd party airports!
If you ignore it, and try to use GSX jetways on 3rd party airports, they will never work, because their positions/parking associations won't match the default airports they were designed to work with, so they will not work and passengers won't use them.