GSX uses those keys only inside editor so yes, it's normal you can't use those while editing a pushback or editing vehicle positions or the airplane points in the airplane profile. However, as soon you exit from any editor, it stops using those keys, so they are given back to the sim.
Of course, before replying, I loaded the PMDG 737-800, used the keyboard to set rudder all the way to the right using the NumPad Enter key, so they rudder stayed at the maxim right deflection, pressed NumPad 5, and it centered, as expected. Repeated the test on the left with NumPad 0, pressed Numpad 5 again, and it centered.
The NumLock light must be visible on the keyboard, otherwise Numpad 5 won't work.