IFD 540 interface to GDC 31-GPSS roll steering

I'm having my Silver Crown radios replaced with a complete Avidyne system to include an IFD 540, 340, 240, and MLB 100.  For good measure I added a GDC-31 for GPS roll steering.  The radios are all in the airplane (a Mooney 231) and the GPS steering switch is installed.  The only issue so far seems to be that the 540 and the GDC-31 don't seem willing to communicate.  I'm not an avionics guy and my knowledge of the way these all talk to each other is limited but here's my current understanding.  For clarity, my airplane has the King HSI and a KFC-200 autopilot.

The 540 and the GDC-31 are currently connected over the 429 ARINC line (through a 121?).  Apparently this is the way the Garmin 530 works and my installer connected it the same way.  The issue appears to be that the 540 will not pass the steering information over this connection.  The HSI is presenting the appropriate information but no turning is happening in the autopilot.  Also, the GPS steering switch in blinking which I believe indicates it's not locked on to a signal.  My installer said he could reconnect over the RS232 lines but that the GPS steering will not be as accurate.  Hmmm ...

What I want to know is if anyone else has this same combination and how it was connected.  If it has to be connected across the RS232 line, what functionality will I lose?  The installer said there was less information or less accuracy, but he didn't know what that meant in the real world.  In the end I just want everything to work without degrading the performance because these two boxes don't play well together.  Any thoughts?

I just got a call from the installer who said he finally played with the settings long enough to get them talking!  And Yes, it was something in the Gama settings.  He didn't tell me exactly what it was; only that it appears to work on the ground.

My understanding is that the 540 is so new that there is no institutional knowledge about it yet.  The GCD guys gave him a deer-in-the-headlights look.  The Avidyne tech guys were equally unresponsive.  I'm sure as these are deployed in greater number that all these odd questions will be commonplace.  I'm just glad the installer was tenacious enough to resolve it without downgrading (?) to the RS232 line.

Two things it seems to depend on - 1. how much previous experience they have combining the two and 2. if they connect using the RS232 or the 429.

I was speaking with the installer again today and he told me he could have connected it via the RS232 line and ti would have worked perfectly and that I would probably never know it.  He said the 429 line has more data and so he wanted to use it (which I also want).  That was the combination he couldn't find an answer to.  Apparently the guys that make the GDC unit told him they get the call frequently but that they didn't know the answer.  Most guys either figure it out or switch to the RS232 line and never call back.  My installer found the answer and did call them back - hopefully it will be simpler for the next guy with the same question.