O que está errado?
$ xrandr --output HDMI-1 --gamma 0.6:1.0:1.6 --brightness 0.76
$ xrandr --verbose | sed -n '/HDMI-1/,/Brightness/p'
HDMI-1 connected 1920x1080+0+0 (0x6c) normal (normal left inverted right x axis y axis) 510mm x 290mm
Identifier: 0x63
Timestamp: 65733000
Subpixel: unknown
Gamma: 1.7:1.0:0.63
Brightness: 0.75
Tags brightness xrandr monitors