I found this question, and that works on my VGA monitor, but my main monitor is connected via HDMI, and xcalib and xgamma don't work on it. How do I reduce the gamma? This is really getting to be an issue because I can hardly use this OS because of how bright the white is.
I had this exact problem and was about to post a question on it, but managed to find an answer.
To find the name of the output. It could be (HDMI-0 or HDMI-1 etc.)
xrandr --output <Channel> --brightness <Level>
Brightness is a decimal number from 0 to 1. For example, this is the command I did to fix my problem:
xrandr --output HDMI-0 --brightness 0.8