It might never grow back to the same as it was previously - I have a couple of axolotls who have a shorter or even a curled gill from nipping damage when they were younger.
Also, younger axolotls heal a lot quicker than adult axolotls, so age is a factor.
Don't worry about it - your little friend may not look perfect any more, but as long as s/he is happy + healthy it's not a reason to stress :happy: