Analogue gamma adjustment

[Rikrok]

I'm not sure if this is the appropriate place to ask but...

I have a screen (WG D9400) where the the midtones (or really anything darker than white) are too dark. I have played with all the screen menu adjustments to no avail. I fed it a signal via an Extron device that has a peak control that can boost the level of the input, and while it does help to lighten the image overall I'm also sending whites past 0.7V which might not be good for the circuitry and it makes the whites on an image bleed blue to the right.

What I'd like is to have a gamma control to lift those midtones up. My searches for analogue colour correction, rgb colour adjust, component colour grading etc. give me nothing, so either I don't know the terminology or such a device doesn't exist.

Does anyone know of such a thing, or if it's not super complicated, how I could make one? I'm comfortable with soldering and basic circuitry.