Unscrew the flux gate from where it is mounted and gently rotate it and see if the reading changes. Don’t tilt it say past 30deg but rotate it through 90 deg as if your vessel is turning. If there are two of you on board you can have one person rotating it and the other calling off the reading. If you are on your own just rotate it, support it and then check the reading.
If the flux gate has failed and they do, you can pick them on EBAY or certain groups on Facebook. If you are mildly engineering competent you can remove the flux gate cover and inspect all of the solder joints and flex PCB. I got to that stage with mine before i discovered it was mounted with ferrous screws, which were actually the cause of the fault.
Do the rotation test and post the result