How to improve the flavor of gluten free challah bread?
Baking is chemistry. Any changes in the formula will have some consequences on the chemistry. It can  be fun to experiment but do not be surprised that the end result is not the same as from the per-modified formula. 

As to your question of more flavor, I do know that increasing the proofing time will help to increase the development of the dough and allow it to extract more of the flavor available in the flour. Try slow proofing your loaf in a refrigerator or a very cool room. 

I do not know anything about the flour in question but I know that the better the quality the  flour the better your bread will be. Avoid flour from  large industrial producers that use large steel mills, these create to much heat in the milling process and deplete flavor, enzymes and proteins. Find a good stone ground miller. You may be limited in the gluten free options.