Now GSX deleted, reinstalled.
Nowhere I said you had to delete anything. Your custom configuration won't be removed in that case.
Stairs don't go to the doors again.
That's because your custom configuration is still active, and is taking precedence over the internal one.
Then edited doors. GSX file with the correct values is created.
You shouldn't edit anything on the PMDG if you are not starting with the correct base configuration.
GSX file write-protected.
This is not normal, of course, unless you were trying to remove it while the editor was still open, you should always have permissions to write in your own folder. Try to remove when the simulator is not running.
Unfortunately, the stairs are in the wrong place again.
Of course they are, until get rid of your custom configuration file which was started to be made when the airplane wasn't supported yet.
The correct files which are included in the current release surely contain the 738-800 and DO NOT require any editing. Be sure you run the FSDT Live Update to get the and be sure your remove your custom airplane profile.
The aircraft editor page should read ONLY "Using Internal GSX database" NOTHIGN ELSE.
If you still don't see this, after being sure you removed your custom configuration, the use the Offline installer linked at the top of this page, because apparently you are not getting the current files from Live Update so, run that one, and don't use the Live Update for the time being. We are keeping the Offline installer constantly updated with the latest fixes.