How to fix it?

Yikes.  That is not normal.  More information from you would be good.  Which P4?  How old? ever crashed? Does this occur on all flights? Did it not happen previously, then started happening after something (like a crash, firmware update, etc).

Obviously, the jello effect is from quick motion.  But you have no quick motion and I notice the jello effect even before you start your pan.  There might be some kind of vibration (prop imbalance?  bad motor?) or unstable flight (like the "bucking" many of us are seeing after the latest firmware update?), or a hardware defect.  Do you notice if your gimbal is vibrating side to side when it is flying?  (Like, maybe a mounting screw or two is loose, or maybe you crashed your drone at some point, or maybe a gimbal motor is no longer strong enough to hold the camera steady).

It would be very interesting to see a close-up video of your drone flying, especially in regard to the gimbal area.  You could do this pretty easily, just hover the bird at eye level and videotape with an extra smart phone.

Is this a new P4 Pro, packing piece behind gimbal has been removed ?

Aardvark Posted at 2017-8-22 06:28
Is this a new P4 Pro, packing piece behind gimbal has been removed ?

(AWESOME question)

I would do a reset of the gimbal....

nd filters. we all learned this back when we had gopros on the phantom 1's.

Please refer the above and offer more details for better assistance, thanks in advance!

See  this thread

https://forum.dji.com/forum.php? ... 24&page=1#pid748001

Don't loose your time trying to find a fix. Many others with these issue have spent a lot of time trying for a fix.
Jello means ... vibrations. Send it back to DJI as soon as possible for a fix.