from what i have gathered, it occurs when larger fish are kept in tanks that are too small to allow them to turn around properly. the repeated motion of backing up flares out the gills and causes them to curl at the edges from water resistance.
as far as personal experience, my black moor has it. he's in a 30g tank, but likes to get stuck in the corners.
I have a gold barb with a curled gill and he seems to be fine. He's had it for months now and it doesn't phase him. He's fat and happy for sure. It looks kind of gnarly seeing the gill all exposed like thaty but it ain't bad.