Go into your Thief folder and open cam_ext.cfg, and find the line:
;d3d_disp_sw_cc
Remove the ; from the start of the line.
Do this for both Thief Gold and Thief 2, they both have this problem and the fix works for both.
This is why I've been saying the patches should just always do this by default, because tons of users have the gamma problem and the solution is extremely non-obvious unless one goes trawling through config files for fun.