I don't think the D5 and the GFX are really competitors. I recently shot the GFX on a day's shoot organised by Camera World (a UK retailer from whom I have bought Fuji gear) and Fujifilm UK. I had the use of the GFX for the day, with a variety of lenses. I have not had time to process the images, but my immediate reaction is that they are of superb quality (the camera's work, not mine!). The camera is a lot like my X-T2 and it was easy to set up and use. Users of the X-H1 would feel even more comfortable.
However, the camera is nowhere near as fast and responsive as a D5. The D5 is a superb high ISO sports, action and wildlife camera, amazingly fast and responsive. But it is not Nikon's best in term of image quality, that is the D850.
I know that Michael Erlewine prefers the D850 to the GFX, but I have not done the comparison myself.