Bruce, an interesting thought, but I didn't check this box. But thanks for the suggestion. It makes sense. And it was almost right, as a checkbox was part of the solution, as I will describe below.
Umberto, you're correct - of course. GSX is looking to the wrong afcad. I read the manual about how to fix this, and set the airport visibility for the problem afcad to zero. Then I turned on the GSX diagnostics, gave GSX a try, but now got an error message. Here's the relevant information from the log file:
Near airport 'zgts' has a visibility range of 0 miles
Near airport 'lgts' has a visibility range of 3 miles
System error while accessing file C:\Program Files (x86)\Lockheed Martin\Prepar3D v3\FSDG\FSDG-Thessaloniki\FSDG-Thessaloniki_1\scenery\AFX_LGTS_closed.bgl: CreateFile failed: The system cannot find the file specified: "C:\Program Files (x86)\Lockheed Martin\Prepar3D v3\FSDG\FSDG-Thessaloniki\FSDG-Thessaloniki_1\scenery\AFX_LGTS_closed.bgl" (system:2)
In the LGTS scenery configurator, I had activated (Bruce, I checked the box) a runway that isn't being used in the real world at the moment. This disabled the file that GSX was looking for as indicated by the error log entry above. I then simply went back to the LGTS scenery configurator, unchecked the box to activate the inactive runway (which deleted the ".off" suffix from the file), and then GSX worked perfectly.
I can live without the inactive runway.
Thanks for the assistance.
Robert