First, you don't say which scenery you use, if it's default or 3rd party.
But, in any case, there's nothing to fix in GSX and that's not an error either.
The request to pushback Left or Right is done ONLY when the scenery AFCAD has defined the pushback preference to be "Both", so GSX will ask. But if the scenery AFCAD has defined only one direction ( either "Left" or "Right", but not "Both" ), GSX obviously assume the scenery designer was correct, and doesn't bother asking you for a direction, when only one direction was defined in the scenery AFCAD.
Now, whether the scenery AFCAD is correct or not, GSX can't possibly know, because it has no knowledge of the airport structure like buildings or other constraints.
Which is why, GSX offers a parking customization feature, so you can specify a different Pushback preference for that parking, and that will override the one in the scenery AFCAD.