xgamma -gamma 0.8
add this command to Startup Application Preferences.
So now every time I login, the gamma is adjusted and the display looks much better.
References
https://ubuntu-mate.community/t/how-to-change-screen-gamma-using-xgamma/12969
https://askubuntu.com/questions/9248/is-there-a-software-utility-to-adjust-screen-gamma-brightness-contrast