DIAGNOSIS: That's expected behavior for this monitor. The 27GP850-B uses an HDR10 standard but doesn't have the high peak brightness or local dimming of a premium HDR display. In HDR mode, it accepts the signal but can't display it dramatically, often resulting in a washed-out, dim look.
FIX: For most users, especially on console, I recommend turning HDR off on the monitor and letting the SDR picture modes do their job. 1) On the monitor, press the joystick and go to Picture > HDR Mode. Set it to 'Off.' 2) On your PS5, go to Settings > Screen and Video > Video Output and turn 'HDR' to 'Off.' You'll get a much brighter, more vibrant image using the 'Game Optimizer' or 'sRGB' picture modes instead.
IF THAT DIDN'T WORK: If you're set on using HDR, try adjusting the in-game HDR brightness/paper white settings. Many games have separate HDR calibration sliders. Crank the 'paper white' or midpoint brightness setting up to compensate for the monitor's lower peak brightness.
HEADS UP: This monitor's HDR is more of a compatibility feature than a highlight. For a true HDR experience, you'd need a monitor with higher brightness (like 600+ nits) and full array local dimming, which this model doesn't have.