This window allows you to change the gamma ramp values for the windowed program. Currently works only for ddraw.
Examples of use:
Some fullscreen games may rely on gamma control to set a dark enough environment in order to have an immersive atmosphere. But hardware gamma control affects the whole screen, so it may be inconvenient to change the gamma ramp values for the surrounding windows and other desktop elements. So, disabling hardware gamma control result in a less immersive game and emulated gamma control can recover the effect. Even when the game doesn't set a custom gamma control, if it is darker than the surrounding desktop it will turn out much less visible than in fullscreen mode. Again, emulated gamma control can compensate.
NOTE: You can also save your custom gamma ramp values and load them with the Libs tab->Color management->Load saved gamma ramp flag.