How to use Indirect Kalman Filter output

Hello.
I would like to measure true degree from IMU sensor by using indirect kalman filter(IKF).
I can calculate error of angle and error of the bias of gyro from indirect kalman filter as attached image.
However, i can not remove the bias of gyro despite using error of bias calculated from IKF like a attached image.
I would like to know how to use IKF's output.
Please let me know if you have any solution.

Which angle are you trying to measure? Heading? Pitch?

What code do you have so far? What does it do? In what way can you tell that it's not removing the gyro bias?