When I turn on the I1, it will calibrate the camera in order to align with the horizon. However, when taking video, the horizon seems always slanted from right to left. Should we always take video with the camera facing forward or let the camera swing at any angle.

DTK, have you done ALL of the calibrations to the R/C, & gimbal? If not, do that first. Then try setting the camera to FPV. After your aircraft is connected to the Go app, look on the left hand side of the screen. Look at the aircraft icon and the green cone touch that and set the green cone to face forward FPV. Hope this helps. Safety First! Ken.