Make sure the F2 Port (A2) is connected to the shutter port of the Zenmuse. Depending on Zenmuse firmware it may be needed to additionally connect one (any) additional port to autopilot.
Then enable shutter port listing at DJI Assistant software.
Disconnect the CAN Bus cable which connects the Zenmuse to the A2 or IOSD.
Controlling the gimbal from RC controller is still possible by connecting the receiver directly to the Zenmuse GCU.