I can only confirm my previous reply. There are no problems whatsoever with the stairs with the PMDG 777, which is an internally supported airplane.
First, check you are using the airplane configuration that comes with GSX, in order to exclude the chance you might have caused a problem with editing it. To do this, use the "Reset" option in the airplane configuration editor, to undo all your eventual edits, then choose the "Restart Couatl" menu option.
Also, try using the latest FSX FULL installer, perhaps you used a vehicles update installer which wasn't updated.
Nobody else has reported this problem, when all the above conditions are satisfied ( latest version, and all customization files removed )