Do you mean the same airplane on a different airport doesn't have the problem ?
Perhaps you always had that problem, but you haven't noticed it before. Your screenshot is not very clear, but it looks like that parking has a jetway.
In that case, you shouldn't see any stairs to begin with, but this is another problem, and it's because you are using an airport (btw, which airport ?) that has static jetways, so GSX can't detect them, and will assume the parking is in the open and will display stairs. You can fix this using the "Customize this parking position" option, and indicate which parking spot have jetways in your airport.
That's only a problem for 3rd party airports that doesn't have true FSX jetways. Default airports and fully compliant FSX airport that have proper FSX jetways (like all FSDT, and many from Flightbeam and UK2000) have jetways detected automatically by GSX, so you don't need to customize them.
Now, the 2nd part of the problem. If GSX is assuming an parking on the apron, you might simply have the doors not properly configured in that airplane, and you just haven't noticed it until now, because you always use parking with (properly detected) jetways. In this case, you should check your airplane configuration using the Airplane configuration editor in GSX, and be sure all doors are set correctly.
