I call FSDT EDDN because was purchased in FlightBeam and the installer left a folder named 29Palms - Library.
And again, what this has anything to do with FSDT ? The whole question doesn't have anything to do with FSDT or GSX.
Your issue is, basically, you have a scenery for an airport from a developer in P3D and a scenery for the same airport from another developer in FSX. This is really a SODE question and, I think this configuration is not supported, since the SODE installation (and the associated XML files) is centralized for all installed simulators, so your situation is probably not supported.
But again, it's entirely unrelated to GSX or FSDT, and I'm not even 100% sure if this could be fixed or not, since we never used SODE from XML files so, you might have better luck asking on SODE support forum.