In the main GMPC thread someone posted a link to IndieVolume which is meant to be able to help. I haven't been around to trying it out yet, so maybe you could give it a go?
I have search here for this and cannot find it anywhere. Is it possibble to get Riderunner to mute or lower the music volume when Garmin speaks turn instructions like it does with IG, since i switch to GMPC I cannot get this to work. If not, then its time for the RR/Garmin experts to start working. Thanks