Kind of useless

Using this cable on the GM55 monitor and the Sony A7iii is mostly useless and definitely a waste of money. You can trigger the camera to start and stop recording from the monitor, but there is no indication on the monitor if the camera is recording or not. If you are using the monitor to start recording because the camera is inaccessible or you can't see the back of the camera you can't know for sure if you are recording or not without looking at the camera. In my testing several times I have thought that I started recording when I was not. Much easier and more reliable to hit record on the camera itself. None of the other functions work. The still button does nothing, the focus button does nothing, and the focus near and far buttons activate clear image zoom. No amount of messing with controls or settings has made any of these functions work. I can't imagine any scenario where this cable is useful in any way for any Sony camera with the GM55 monitor at firmware version B0.3.45