If you have the ati driver installed correctly and have xorg setup correctly, you can easily change the brightness, contrast and gamma on linux.

I could not find anywhere on google a single search result except for setting gamma, so I post this here.

To change brightness and contrast you do this through aticonfig. These settings are saved as you change them and are irreversible without deleting /etc/ati/amdpcsdb first. I recommend making small increments and make sure your syntax is correct.

First you need to determine what monitor(s) types are connected. With aticonfig in terminal try aticonfig --query-monitor. If you get the error it is not supported when RandR is enabled. You can then try xrandr. This will return the screen # and types. From xrandr you should be able to pick out what display type you need for aticonfig, the naming will be different.

aticonfig requires the following display type options in the string crt1, lvds, tv, cv, tmds1, crt2, tmds2, tmds2i, dfp3, dfp4, dfp5, dfp6. So you can try one of these until you get the right one. Querying incorrect display type does no harm. Typically, lvds is going to be your LCD.

To determine the contrast and brightness you are using type the following below in a terminal, lvds is the display type and in the output, value is the current setting;

Contrast
aticonfig --query-dispattrib=lvds,contrast
Which outputs for me:
contrast attribute information of monitor lvds :
default:100, value:85, min:0, max:200, step:1

Brightness
aticonfig --query-dispattrib=lvds,brightness
Which outputs for me:
brightness attribute information of monitor lvds :
default:0, value:-3, min:-100, max:100, step:1

Notice how brightness is min value of -100 and max 100 while contrast is a min value of 0 and max 200. So be careful.

To set these, be very careful of typos or accidentally setting a null value. Your screen could very easily go black:
aticonfig --set-dispattrib=lvds,contrast:100
Where 100 is the contrast value you are setting. And the same goes for brightness:
aticonfig --set-dispattrib=lvds,brightness:0

UPDATE: Use fglrx_xgamma for setting gamma. Xorg.conf gamma no longer works in fglrx versions 9.5 and later.

Gamma is not supported anywhere I can see in the man for aticonfig. So for gamma you can see the current setting through xserver by typing xgamma. It should return values for red, blue and green.

xgamma supports decimal values like 0.85 or 1.0. 1.0 being default. If you want to change the gamma for all 3, type xgamma -gamma 0.85 or whatever value you want. If you want to change individual colors, you have to do it one by one. Replace -gamma with -rgamma, -ggamma or -bgamma. Red, green, blue respectively.

The thing about xgamma, these settings are only stored and saved for your current user/boot session. To make these settings permanent, you will need to edit /etc/X11/xorg.conf. If your xorg.conf is not setup, you should get started reading here.

So open xorg.conf as sudo in your text editor of choice. For gnome users, one of my fav commands is gksu gedit /ect/X11/xorg.conf &disown.

Next goto Section "Monitor". Inside this section simply add your gamma setting with, Gamma 0.85 0.85 0.85 (red, blue, green). Or you can put a single value for all 3. Save and exit.

That’s it, pretty straight forward I hope.