I just got done testing this on my machine. To get the default gamma, change the three variables so they read as follows:
seta r_ext_gamma_control "1"
seta r_ignorehwgamma "1"
seta r_gamma "1"
"1" is the default gamma setting for the game, but if you need it higher, try using 1.2 or some decimal. It seems to be in very fine increments.
Another thing-- if you want to do it mid game, bring down the console by hitting shift+tilde, then type:
After that, typing the command "vid_restart" will reload your current map with the new display variables.