Reset/change gamma and brightness after Diablo II mess

Use xrandr to list your outputs:

$ xrandr
Screen 0: minimum 320 x 200, current 1024 x 768, maximum 8192 x 8192
DVI-0 connected 1024x768+0+0 (normal left inverted right x axis y axis) 304mm x 228mm
As you can see my output is DVI-0 to change the brightness:

xrandr --output DVI-0 --brightness 2
To change the gamma:

xrandr --output DVI-0 --gamma 2:2:1