was having the same issue, actually still am. Used two different video cards, Nvidia 1060 3 gig and AMD rX480 4 gig. Loaded COH on a different machine and gamma worked for both video cards. Copied COH installation to same different machine, and Gamma still worked. Both machines had the same motherboard, amount of memory and CPU.. Head scratcher. Only difference was the machine that Gamma didn't work also had dual monitors, I was using monitor1 according to COH options.. Tried it on monitor2 , no Gamma..
So. I powered down the machine with dual monitors, disconnected monitor2 and powered back up, loaded COH and .... Gamma worked as long as the second monitor was disconnected at system boot.
So my issue was that Gamma adjust doesn't work with dual monitors, even with different video cards.. Hope this helps..
