Awww... what a cutie!
I don't really think anyone has come up with a way to prompt gill growth, other than offering good healthy food and a clean, good environment and seeing what happens.
A lot of it is simply down to the genetics of the axolotl, just like us humans all look different, I guess.
I also believe that if the environment your axie is in is low in oxygen, he or she may grow longer gills to try and help absorb more oxygen from the water.
Ultimately, they are all unique, we just get to love our axies for how they are ^_^