ohhhhhhh, why do you make that question so difficult? It is so easy to reply if you if you know the world, Therefore I reformulate the question differently:
if you and not another guy, put two cameras (GH5 and a full frame camera) on one same tripod and you make a UHD video, which video of both is better? Wich has a better image quality?
With "VIDEO" not "PHOTOS" it is harder to see the difference in many ways. The GH5 has the advantage of longer than 29minute videos. The GH5 also has good auto-focus in video (the guys using full-frame use follow-focus devices for smooth manual focus even if the camera has good video AF). And while good video AF speed won't matter to experts, it matters to 99% of the people buying these things.
For those 99% the gh5 is probably the best video/still hybrid.
There are still things a full-frame can do that a m4/3 can't. To most it won't matter.
A full frame camera has considerably better dynamic range. These more variations in color, more details in shadows and more details in the brightest areas where on a smaller sensor (or older full frame) there would be nothing.
There is depth of field control. A full frame camera can achieve shallower depth of field. This is the blurring of the background. For example her eye is in sharp focus and then the scene behind her gradually swims in to focus without losing focus on her.
There is light gathering. A larger sensor has more photons hitting it. Better in low light. Less noise.
Finally there is lens selection. Poor optics = poor results no matter what the sensor. Full frame cameras, particularly those from Nikon and Canon have the best lens selections especially at the high end.
I am not recommending a full frame for video. Generally those who should go that route. The GH5 is a phenomenal camera. It has very good support in video accessories. It is comparatively easy to learn and easy to use. The older gh3 and gh4 are also still great.
I use a Nikon full frame for photos and a Sony rx10 and Panasonic gh3 for video.