In my testing, I am powering my gimbal with a Turnigy 2200 mAh 3-cell 11.1v LiPO battery. The advantage is that the gimbal will not drain power from the P2. The other advantage is that no soldering is required. The disadvantage is that the battery is fairly heavy (at least compared with the gimbal and my Xiaomi Yi camera). I "secured" the camera to the gimbal with rubber bands, and it seems to work well on my test bench. (I have not yet tried pitch control.).
Once I log a few more initial test flights over the next 2 weeks with my P2, I will mount the camera/gimbal/LiPO battery on the P2 and see what aerial video I get. After that, I will crack open the P2 and connect the pitch control cable from the Naza to the gimbal.