Did you try holding the C2 button on, and turn the photo wheel on upper right? You can adjust it that way, but I'm unsure if it will stay that way in your case.
In January I crashed my P3P. Fortunately the crash didn't scratch the camera or gimbal at all, I had a camera guard installed, along with the gimbal protector from Killer RC. However everything else was really bad, replaced 4 motors+props, complete body shell, landing gear, battery tray, and one camera cable. Anyway, $300 later I get it back and my gimbal won't stay level with horizon. I calibrated the gimbal several times, no joy. Calibrated IMU, no joy. It was puzzling. Finally I went into all the gimbal settings and tried to touch every setting I could find that was gimbal related. I mean I looked at everything, adjusting to extremes both ways, every setting possible. FINALLY it straightened itself out. Unfortunately I adjusted so many things, I'm unsure what it was that fixed it. It just magically started working and it's never given me a problem since, even today, rock solid.