Try to enable logging and check if there's an error in the %APPDATA%\Virtuali\Couatl.log file when it appears stuck.
I saved a flight and reloaded it. After asking for boarding, GSX freezed again. Here the error that I can see on couatl.log:
File ".\GSX\assistanceServices\baggageTrain.py", line 594, in baggageGrabbed
AttributeError: 'TrainOfVehicles' object has no attribute 'setWagonBaggages'
{'Airport': 'EFHK', 'Requested assistance services at': 'Gate 28', 'User Pos': (60.31873333324952, 24.965397222258858, 58.9997 m, 4.44072 m, 137.43143861173812, 0.006023351568728685, -0.0, -0.0, 4.572, 1.0)}
Nothing else.