The weight of the gimbal probably exceed the weight that the Phantom can safely lift.
Adding a camera makes the weight overload worse.
You need a way to power the gimbal and camera as well as communicate with it.
By the time you have all that figured out, you could purchase another vehicle that has the ability to use another gimbal and camera setup.
The Zenmuse H3-3D weighs 168 grams. You would still need a way to power the gimbal and camera. I'm making that comment based on DJI Phantom 3, not being able to swap them over to being able to use the GoPro camera.
Before going through the trouble of finding a way to power them, I'd look for a P2 that has the LightBridge already installed. If you want more power for flight, you can purchase the P3 motors and props.