Place the bird on a level surface while videoing an object that has a horizontal plane. Do this while observing the screen to see if there is any tilt.
I actually operate with the "Gridlines"on so I can check horizontal positioning thru the camera/screen before videoing.
As mentioned also try again the gimbal calibration and if all else fails adjust the - and + on the camera/gimbal roll( top right header HD icon then the gimbal icon in the bottom left of the just opened menu screen then select adjust gimbal roll and then select - or+ as appropriate to get everything horizontal)
If it is bad after that, it is outside of any adjustable parameters and needs servicing in my opinion..