Could be a driver-related issue - might want to try reinstalling your graphics card drivers to see if that fixes the issue. Here are a few things you can try to reset your gamma:
1. Load Task Manager and find Windows Explorer, right-click and select restart.
2. Run EQ again and click Decline on the EULA dialog.
3. Download and install
f.lux - load it after you've exited EQ and you're still experiencing gamma brightness.