Just my two cents, if anyone has issues with color noise such as too much red or too much green at the pixel level you should probably run photoshop remove color noise on all your upscales. Im guessing the horizontal banding issues still exist in GPU mode, it also helps a bit to remove those, especially if you put the remove color noise(in photoshop) at 100%. CPU mode wont have those banding issues from what ive seen and is superior quality. I think there should be an answer as to if it will ever be possible for GPU mode to match CPU mode in quality.
Another issues is that switching to the manmade AI option in CPU mode is sure to crash gigapixel for me. On my Ryzen 1700X machine.