GSX *used* to have more limits on some operations, but we REMOVED a lot of them (most related to refueling), because users asked for it.
So, now it's possible to refuel with passengers on board (with the appropriate procedural warnings, such suggesting to turn on no-smoking signs, and turn off seatbelt signs), or refuel while boarding.
However, catering while refueling is still not allowed, and it was never allowed before.