GSX completely releases the airplane after it completes the Pushback, by sending the standard "unfreeze" command to the flight model.
If the airplane has extra custom systems (like an NWS with a bypass pin simulation), that detects GSX is pushing and are supposed to detect it stopped pushing, it's the airplane responsibility to reinstate them to the normal situation by detecting GSX has stopped pushing, in the same way it detected when it started.
If GSX crashed in the middle of a pushback, a subsequent restart would also unfreeze the airplane, but if this happens and the airplane has such custom system that should be reset as well, then it's a matter of the airplane code that should be able to detect a GSX restart, which can be done fairly easily by checking those freeze standard variables. That, assuming GSX *did* crash in the middle of a pushback, which is the only possible reason I could think that might not be covered by the airplane code, assuming it *does* need to reset those custom systems.
