In my environment, Mi fit and Gadget Bridge could not coexist.
Specifically, with Mi Fit installed on the phone, it was not possible to connect or update MB3's firmware with GadgetBridge.
Gadgetbridge shows that it is connected, but the mark of the band is displayed in gray, and it was impossible to update even if opened the fw file.
Once disable Mi Fit and restart gadgetbridge, the mark of the band becomes blue and the firmware can be updated.
I normally disable the gadget bridge, and when changing the firmware, disable Mi fit and activate the gadget bridge.
I'm glad if you can use it as a reference.