We are beating a dead horse and perhaps arguing semantics. But I think it is important to acknowledge the distinction between "heading" via a compass and COG, via the GPS. They are not the same and not interchangeable although in most cases they should be close. I have an extensive Garmin system and am very familiar with how it operates. First, an MFD/plotter does not require any device that provides electronic heading information unless you want to use features such as MARPA or Overlay. If you want heading, you absolutely can NOT get "Heading" data from the MFD/plotter/GPS and it will NOT act as a compass. You'll need a actual compass, usually the AP in most set-ups. Otherwise "heading" field will be blank on the Garmin display. This is my issue with the term "Garmin Compass". There isn't one. We are all speculating on the OP's setup but my reasonably informed opinion is that when he refers to his "heading on GPS" he's likely referring to his COG which roughly matches his helm compass as one would expect. What's not matching is his AP heading display, likely a calibration or interference issue as we all seem to agree. But that mismatch has nothing to do with their ability to interface since they could be completely detached from each other and should still approximately agree.