If you want to set the gamma for multiplayer, you must use jk2mpconfig.cfg and play with the gamma variables.
Otherwise, jk2config.cfg is only used by the single player exe.
I think if you keep playing with those gamma vars, you will get results.
Maybe try this:
seta r_ignorehwgamma "1"
seta r_gamma "5"
I'm not at home to test this, but I will get back to you soon.
PS- There is also a brightness setting in the advanced display options which might help.