Does anyone know the requirements for how much of a challah must be
made of the "five grains" (rye, spelt, barley, wheat or oats) in order
for it to fulfill the commandment to eat challah? And how this would
translate into the mitzvah of *taking* challah? (I.E. Would you need
to separate more from the challah dough to burn?) This is a question
I'm asking on behalf of myself and also another celiac who are
interested in being able to fulfill these mitzvot but can't figure out
how much of the challah recipe would need to consist of GF oat flour
in order for everything to be halachically correct. Is anyone
knowledgeable about this, or has anyone asked such a question of a rabbi?

Thanks