There are a lot of threads about tilted horizon here. I have tried some of them but I still have to a) Adjust Gimbal Roll or b) fix the horizon in post (shooting 4K helps because I output HD).
I have saved some snippets as a memo:
IMU and Gimbal calibration:
Might fix tilted horizon. Make sure "Adjust Gimbal Roll" is 0° before calibrating.
Cool the P3 (and battery?) for one hour in my refrigerator or other cool place followed by an immediate IMU calibration and gimbal auto calibration on a perfectly level surface on power-up as soon as the DJI GO app shows a live camera view from the P3 (which will be very foggy due to condensation on the cold camera lens).
Gyroscope Mod 0.0-1.5, Acceleration (g) X, Y 0.00 Z -1.00 Mod 1.00 (0.98-1.02)
If the horizon is still tilted you might try the following:
Get airborne, climb to a height that gives you a clean horizon and then manually change the gimbal roll in the DJI GO app > Gimbal Param Settings > Adjust Gimbal Roll lets you adjust the horizon in 0.2 degree increments. You might make this a P3P shortcut to C1 and C2, then turn right thumbwheel.
A poster has suggested tilting the camera straight down pointing at ground before starting the auto gimbal calibration.
A poster has suggested doing the gimbal calibration while the AC is in the air on a calm day with little wind.
A poster has suggested that if the AC takes off from an un level surface the horizon will be tilted.
A poster has suggested that the camera is attached to the gimbal unit and secured by a small set screw. It is possible to loosen the screw and rotate the camera assembly slightly to correct the tilt.
A poster has suggested doing IMU calibration (as above), but then adjusting gimbal roll with the grid on the camera to some level object, followed by gimbal calibration.
Tilted Horizon issues on Phantom 3 pro
Non-Level P3 Pro Camera Horizon FINALLY Resolved!