Make sure Zubax GNSS and PX4 the autopilot are aligned (use arrow markings) as shown below.
To check the number of barometers on board. Open the tab
FLIGHT DATA then select tab
Status. It will appear that two barometers are detected. One barometer is on PX4 the autopilot and other is on the GNSS module.
To set other barometer as the primary one. Change the parameter
GND_PRIMARY to 1.