I have bought both of them and now i'm trying to decide which one I'll keep. So far it's very tight tie. Both of them have fair amount of backlight bleeding, i would say on gigabyte it is more pronounced, especially on top left corner, but it is mostly visible in dark room and on black backround only, so thats no dealbreaker for me.
Next thing i have noticed is that iiyama has slightly worse viewing angles. When you open for example excel spreadsheet, on iiyama is color shift on both sides of screen slightly more noticable than on gigabyte. But again, this is nitpicking, you won't notice it in games.
Iiyama has slightly better contrast, max brigjtness and more vivid colors. I dont have color measurment tools, so i can't tell which one has more accurate colors. Both of them have noticible pixel inversing (or pixel grid) visible with higher refresh rates. In term of speed, they are pretty similar, when you set correct overdrive..
Regarding flickering-i have gtx 1660 and i have connected both via DP. There was minor flickering on loading screens with some game, but so far i haven't noticed any flicker in games. Interestingly, the flickering was totally the same on both. Same intensity, same timing etc.
So thats basically it, other factors to consider is that gigabyte doesn't have usb hub and has smaller hight range on adjustable stand.