r_ignorehwgamma 0, is the cvar you are looking for
after setting that to 0, try playing with this cvars to change your brightness,
(you will need a vid_restart to apply the changes)
Code:
r_gamma 1/2/3
r_intensity 1/1.5/2/3
r_mapoverbrightbits 1/2/3
r_overbrightbits 0/1
r_ext_gamma_control 0/1
But be aware that modifying the overbrightbits cvars may be considering cheating since they add a lot of bright to the textures and the playermodels are easier to spot
also if you search those commands on google you will find more info about them on some quake3 and other games forums