For some unknown reason, there have been multiple reports of people getting a grayscale screen with GZDoom. This happened as a result of the removal of the display gamma ramp setter, which is all done by a shader now, instead.
The fix is very simple - simply open the console and type this:
I have no idea why some people's INI's defaulted to 0 and some defaulted to 1. But there's the fix for you.