Monitor the value GearShifting in the Vehicle channel of the Data Bus in the FixedUpdate cycle:
https://vehiclephysics.com/advanced/databus-reference/#vehicle-channel
When GearboxShifting goes from 0 to 1 then the gearbox has started a gear shift operation:
When GearboxShifting goes from 1 to 0 then the gearbox has completed the gear shift operation. The value GearboxGear contains the new gear number.