Game balance is difficult I agree, good luck and hope you figure out something. I will be checking up on this game as I do want to see it grow and update!
About Gaiamon, you're right there's no image. It's a super cool digimon that effectively no other game has due to obvious balance reasons. Probably my personal favorite, ever since I saw it playable in a fan interpretation, in some obscure fangame.
I understand official art is a solid bar to have though. There is some official looking fan art of Gaiamon but it'll never be the 'canon' Gaiamon.