How does cubepilot disable the barometer height?
How does the altitude data source of UAV switch to GPS
I think changing EK2_ALT_SOURCE to 0 or EK3_SRC1_POSZ to 1 should do the job (based on which EKF you are using).
But, why do you want to do that?
Most of the time barometer outperforms GPS, for altitude reading purposes.
Sudden jumps might occur if you just rely on GPS as an altitude source when you have a GPS glitch.