poyHaving a similar issue. When I alt tab, the game just becomes darker but the in-game gamma is still the same; switching from 1.6 to 2.6 and back to 1.6 fixes the issue until I alt tab again. (Disabling fullscreen optimization did not change anything as well)
Yea this is the bug I'm talking about, still no concrete solution. Tried everything in nvidia having to do with gamma, tried rolling back my drivers a bit, no dice.
I still think it has to do with the variable refresh rate options or something else randomly hidden. I ended up just living with 2.2 gamma for now but one thing I didn't try was disabling gsync and everything related with that, so if that's the case I guess you have to decide what you care about more.
Unfortunately I won't have a pc to test that theory for about 2 months, so someone else might have to try and confirm.