All the other Gelatin challengers seem to have Achievements that can be earned but I don't see one for Gelashell. Will there be one?

There already is one! It's called Turtle Soup.

Defeat Gelashell 10 times!

:)

There is one. It's called Turtle Soup, but there's only one for beating him 10 times. No tiers or prizes or anything. It's on the second page of the battle section of achievements.

Thank you Tobiko, must be blind - but I'm glad it is there! Thanks Archer, looked right past it - glad to know it is there as I'm about to fight him for the 10th time!

EDIT Unlocked Turtle Soup!!

Defeat Gelashell 10 times!