There is a software bug in the G3X (v9.15 and earlier). According to the AFMS, if the GMU 11 fails the G3X is supposed to display a red X over amber HDG and display magenta GPS TRK on the HSI. But, instead, it continues to display white HDG as normal. What is happening is that the logic is not switching to TRK upon GMU failure and the ADSU 25D is continuing to compute HDG but without GMU input so the heading will drift over time. The G5 does properly switch to TRK. I don't have a GI 275, so I don't know how it behaves. I reported it to Garmin and after some back and forth, Support was able to reproduce it and passed it on to Engineering for resolution.