GSX doesn't know anything about the outside world, except the AFCAD data. It cannot possibly know if there's a building. Also, it relies on the scenery developer to prevent a pushback direction that would end up either inside a building or in front of it.
Which is why, it allows you to:
- Override the parking directions as specified by the scenery developer, preventing a Left or Right choice that shouldn't be used
AND, if that's not enough:
- Define your own Custom Pushback nodes.
It's all explained in the GSX manual, of course.