Go to your \q3test\demoq3 directory
and open up q3config.cfg with a text editor.
make sure the line that says
seta r_ignorehwgamma "1"
is equal to to one as shown in the above line.
Then change the line that that says:
seta r_gamma "1.6"
to the gamma you want (1.6 works great for me).
A side effect is that you can only change the gamma through the .cfg file, but once you have your settings teh way you like, you dont have to change it agian.