As for setting the FY-G3 gimbal up with APM/pixhawk you can see how to do that in our FY-G3 Manual
But the short version is here:
You simply need to connect the FY-G3 to one of the APM outputs, and then specify the channels within the mission planner, but just make sure to disable the stabilise tilt box. But as you want to use the ROI feature on APM its best to route through APM in case so that it can control the pitch to keep a position in view as you fly around it.
Otherwise you can also just connect the FY-G3 directly to a spare channel on your receiver and bypass APM if you dont need to use the ROI feature.