Try the xgamma -gamma red green blue command. Open a terminal and do the following:
xgamma -gamma 1.2 1.2 1.2
You might want to try with higher or lower values to see if that helps. Once you've found the correct brightness for your display, you can either modify a line in your /etc/X11/xorg.conf or create a little application to start when Gnome starts. I help you with those if the above command works for you.
Regards and welcome to LQ.org