So I just joined the forums specifically for the same reason. I will jump in on your post with some more info, hoping it will help anyone else who has this issue. I have used GSX on P3D for a few years with no issue and have since moved to MSFS and using Prosim B738 flight Model. I can say that GSX works OK except same as you it will not save any changes I make, so my pax climb into the plane via the cockpit and bags go into Engine 2!
i.e. I can configure all of the doors however GSX won't keep my settings.
I deleted the airplane virtuali/airplane folder and ensured GSX creates a new airplane folder and the gsx.cfg file which it did with default values. C:\Users\username\AppData\Roaming\virtuali\Airplanes\prosim_b738 - I have full read/write permissions to this folder.
I have tried a number of ways to set the doors, namely PAX1&4, Cargo1&2, Service 1&2.
1 - Initially using the 3D config gui.
2 - using the airplane config screen to get the 4 position values for each door. I then made a copy of gsx.cfg and then manually entered these in the gsx.cfg file using notepad.
3 - using the airplane config and clicking on each door, I typed these into the field boxes.
4 - someone somewhere suggested putting a copy of the gsx,cfg file in the Community/airplanes folder. No good either.
I restarted coutal service every time, and in some cases restarted prosim and msfs, all to no avail.
I must be missing something.
Here's hoping.
DrF