I applied the patch to linux 5.2 (among
0001-drm-amd-display-Allow-fast-updates-again-for-swappin.patch) and as
expected, gamma adjustments have stopped working.
Unfortunately, the games still show the issue.

Should the debug information be contained in dmesg? After doing
echo 0x4 > /sys/module/drm/parameters/debug
and starting Hitman 2, there don't seem to be any comprehensive debug
information inside the dmesg log, at least not to my layman eyes (attaching
right now nonetheless).