Connect the Seagull #REC to the relevant AUX ports on the flight controller and then map the switch on the Tengri Ground Control unit to the relevant command on the Seagull #REC using the Set_Servo command on the Tengri Ground Control unit.
For example to start recording video on your camera, connect the channel 1 cable on the Seagull #REC to the AUX1 port on the flight controller (which is Servo 9). Then go to the joystick setup option in QGC, select a button you wish to map, select the Set_Servo command for the button and assign a value of 1800 [as stated in the Seagull manual] to Servo9. Pressing the button that was selected will now start the video recording on your camera. Any of the Seagull#REC commands can be easily controlled in the same way.