Hi - firstly, I have to say I have never kept this species of fish so don't have first hand experience of treating them. All I know is that they are quite sensitive, especially to certain medications, so treat carefully and double-check anything you use beforehand.
Two things spring to mind, looking at the photo. The first is that this could be mouth rot (Columnaris). Does it look fluffy at all? Hard to tell from the pic. Then again, Columnaris isn't always that fluffy.
The other thing that it could be is a bacterial infection causing the skin to flake off. I can just make out some sort of black and white gravel in the pic - is that the white coral type of gravel? If so, that is usually quite rough with sharp edges. It could be that your fish has been scraping his mouth when feeding, leading to a bacterial infection.
I think if you can take a closer inspection of its mouth to ascertain whether it looks to be something growing around it's mouth (Columnaris) or whether it is skin flaked off, then you will know which course of treatment to look for.
If you are in the UK there is Myxazin which will treat mouth fungus and any other bacterial disease, so if that medication is ok to use with a BGK, I'd suggest that as a starting point. The manufacturers do say that it can be used also as a brush-on application - but, again, check to make sure that a BGK can cope with that on their delicate skin.
Regards, Athena