If the horizon is not 100% leveled but remains stationary as you operate the tilt control you need to re-calibrate it:
- Power up your Disco PRO
- Connect USB
- Start the SimpleBGC tool Version 2.1
- Hit connect
- Hold your gimbal 100% straight and try not to move it during the calibration process (press your gimbal to a flat surface)
- Hit the button Calibrate ACC in the SimpleBGC interface
- Hit the button Calibrate Gyro

That's it. Your gimbal should keep now the calibrated angle. Cycle power and make sure that's the case.

When your gimbal still tries to move to a certain angle and holds that angle, try to disconnect Channel 5 and 6 from your top plate. If that returns the Gimbal back to a level position the problem then your remote control is not set up properly and it is sending a wrong signal on channel 5 and 6. Make sure whatever switch is assigned to your channel 5 and 6 is centered, then cycle power of the DISCOVERY PRO. 

If your gimbal drifts during flight or specially when yaw your copter, try the above steps first to see if the issue is resolved. If not, 
- Power up your Disco PRO
- Connect USB
- Start the SimpleBGC tool Version 2.1
- Hit connect
- Invert your roll motor (tab: motor configuration). if it's already inverted, deactivate the inversion.
- Hit write 
- Select Profile 2
- Invert your roll motor here as well. if it's already inverted, deactivate the inversion.
- Hit write

Profile 1 is used while on the ground. Profile 2 is used once the current consumption raises above 3A (usually as soon as you're about to take off). That's why you need to change the settings on both profiles, but it's especially important to update on profile 2 since that's the profile used in flight. profile 2 will emit a squeal, this is normal. Try this setting and see if the leveling characteristics improve.