I've owned the GR20/GK3 in the past, TWICE.
The GM800/Gk5 combo has better tracking (NOT perfect, far from it, it's just the nature of the beast, triggering sounds via hex pups) and less glitches in general.
In my humble opinion, this has nothing to do with the Gk5 vs Gk3 debate as a hex pup, is just a hex pup, a passive component. It has to do with the fact that the GM800 is a computational beast in comparisson.
More that 1000 sounds available (and as always, not all are good), great customization, small size but it doesn't have a built in expression pedal which is good in terms of modularity and size, but bad because chances are you will need one and that means you have to pay for it.
Great PC editor via USB but a bit slow for my tastes, it gets the job done.
The front panel menu reminds me of digital devices 20 years ago and for me it's not intuitive either. So, thank God, the editor works.
Kudos to Boss for moving away from the 13pin cables, which to me were just a PITA in terms of reliability.
To update the firmware you need a USB flash drive, which begs the question, why Boss didn't just gave this device the capability to update the firmware via the USB cable.