Depending on the version of simpleBGC board you have, 1st check if the board version has the latest firmware that supports MAVLINK.
Choose a usart on the simpleBGC, preferably not 0 as this is used for the USB but can be used.
From the simpleBGC USART tx pin connect to pixhawk RX pin 3 of TALEM 1 or 2.
From the simpleBGC USART rx pin connect to pixhawk RX pin 2 of TALEM 1 or 2.
From the simpleBGC USART GND pin connect to pixhawk GND pin 6 of TALEM 1 or 2.
From GCS (mission planner) goto full parameter list, search for MNT_TYPE set to 4. This tells ardupiot you are wishing to use MAVLINK to control the gimbal.
If using talem 1:-
Set> SERIAL1_BAUD =115
If using talem 2:-
Set> SERIAL2_BAUD =115
Now, what channels from the RC are to be used for control?
so for example rc10 for pan, rc11 for tit and rc12 for roll:-
RC10_FUNCTION = 12
RC10_FUNCTION = 13
RC10_FUNCTION = 14
Now you can do lots on interesting things within mission Planner
Right click on map select point camera to here, then enter altitude.
The camera will face and 'lock on' to that point.
In missions there are do_mount controls
There is also ROI, Region Of Interest
In mission planner, flight data, Actions, the bottom left drop-down changes the gimbal control mode. RC Targeting = manual control.
There are other perams that can be set like:-
There is an issue though, as above in RC Targeting the gimbal always yaws with the copter.
hope this helps