The issue is with the adaptor, it will have the wrong value resistor in it which is telling the Git2 that it has a mono microphone.
As far as I know there is not yet a good list of what works for stereo, what works for mono and what doesn't work at all. I don't even know if the official GoPro one works, but it should do... If you do some searching on this forum you will find a lot of posts about it, hopefully someone can recommend something, although I have two from the same source, apparently identical but one works perfectly while the other looks like it came from a different factory and does not work at all!
When you get it working, please post some advice for others...