There is no cure... just provide good water quality and possibly you might help it some, some people do gill plate trimmings for the curled portion too but that's a risk if you don't know what you are doing...
Sent from my SAMSUNG-SM-G870A using MonsterAquariaNetwork App
Should live just fine just keep your parameters to par, for some people they just think it's unsightly to look at. He might be more sensitive to water parameters because of this
Sent from my SAMSUNG-SM-G870A using MonsterAquariaNetwork App
AFAIU, the current thought by professionals is that gill curl has little-to-no bearing on fish's quality of life.
It could be genetic or from bad water. If from bad water, it can right itself in good water (long term), good diet and overall care - proper space, proper oxygen, etc.
Some people resolve to a minor surgery, as mentioned, or a message and stretching. I'd not do either.