Interfacing with the camera. The camera on the mount right now is a Nikon- which has a proprietary plug called an MC DC 2- which carries remote focus, trigger and GPS. A quick check on amazon brings up the bare cable for around £5 same day delivery, OR a longer cable with a hand remote for the same price. Awesome. Don't expect amazing quality but the wire is what we are after, and the more flexible the insulation the better- lest you have to strip the insulation to allow your gimbal to move freely. The one I've linked above is flexible enough. Camera buttons are very simple circuits- like those in your car. They rely on a signal voltage being connected to the ground of the device- and that closes a circuit and tell's an IC that a button has been pressed. Armed with that knowledge you know that you can open your camera and apply this relay method to any button for full breakout PWM control. Just remember to bin that warranty card- it won't be any use anymore.
Cut the remote shutter cable with enough slack to allow full movement of the gimbal but try to leave some sticking out of the hand remote so it remains useful for the future. Relays! Get solid state relays, amperage rating doesn't matter. The lighter the better. I've used ones from DIY Drones before and numerous clones. They should cost between £15 and £20. The ones I used in this build were from Mr-RCworld, a UK company who make their own products and even supply datasheets! Nice work.
We are only interested in the pair of terminals without the red dot- the red dot pair are commoned all the time, the ones without are switched with the relay. Use a multimeter in continuity mode to determine which pins on your connector go to which colour cable- there is NO standard with clone products such as my fancy remote shutter button. Turns out white is ground (obviously?) Yellow is focus and red is shutter release. Wire as per my picture- I've commoned the single ground wire over to both relays. Remember to strip and tin those wires before clamping into the terminals for a more effective grip and a more reliable build; we aren't Space-X but there's no excuse for shoddy electrical work failing at altitude during a mission.