Yes on linux you use xgamma like this
xgamma -gamma X.X (x.x being 1.2, 1.4, 2.0 or whatever you want)
Although for me it had a habit of slipping back to 1.0 after about 10 mins, i dont know the cause so YMMV.
What about setting the gamma in .tremulous/base/autogen.cfg, Is that any different?
Mine is seta r_gamma "2.4"