I think he means RDR is not mastered for HDR?I don't understand why people buy a HDR monitor without knowing how to use it. Or it is just another troll attempt to bash down the best HDR FALD LCDs that outperforms any other OLED or any other monitors not capable of true HDR.
Though RDR2 is not graded specifically for HDR, the game already gives HDR calibration for white points up to 500nits, and HDR brightness up to 10,000nits.
With white points 500nits and HDR brightness 1800nits. The game looks good without loosing details, better than any other monitors.
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My recommendation: Not a game but try Dune in 4K Dolby Vision/HDR10. I watched on this monitor and it is revelatory. I played it side by side with an OLED and while latter shows just marginally better contrast (hair-splitting) in dark scenes, XG shows the bright details like lights much better and makes them pop more life-like. The outdoor scenes such as in the desert sun were extraordinary and looked realistic and the OLED could not compare at all to the XG. It wasn’t even close when the hot desert sun was blazing.
For game, maybe try Elden Ring.