I ride with Android Auto tuned on 99% of my GW riding time. The other 1% of time my phone has over heated and is busy trying to melt the side case...but I digress.
Anyway, I have GPS running all the time. I have never noticed any deviation that makes me suspect speedo error. I'm pretty sure I would notice. My commute is on an interstate that requires speeds that would expose an error. It's never even crossed my mind. And I'm a guy who has used SpeedHealers in the past.
The main thing about the GW speedo is it is not on the TFT. I found that odd, even bizarre at first. Now I understand. I've experienced enough TFT lock-ups, crashes, and random "you can't do that while riding" message frozen on the screen, that a TFT speedo would be a bad idea.