I suspect it's old sensor tech in the GX8. The GX8 also uses only part of the sensor for 4K. In fact, the GX8 crop is even greater than that of the GH4 for 4K video.
Seems that the G7 is a stronger proposition for video then:
- 1.1x crop in 4K, 1.25x with the GX8
- 3.5mm microphone jack, 2.5mm on the GX8
- Two thirds of the price
- No information about compatibility with the 0.64x speed booster yet, but the G6 supported it while the GX7 didn't (the GX1 did)
If m4/3 were still my main setup, I would buy both, one with the main emphasis on photography and one for video. Total weight is 900g (less than my D810) and they use the same batteries. "Unfortunately", that will not be an option for me. The Nikon can do things that these can't, and monies aren't available in unlimited quantities. I must say that Panasonic has produced two excellent cameras here though, two of the best at this moment in time