Well, nothing. I just figured it was a given that "this is Rachel's gluten-free food, so eat your gluten-laden brownies before you eat hers."
I think you've found your problem! I have found in my life that people are well meaning, but you must communicate your wishes to others - they can't read minds, and dont mean to disrespect you!
It may also help to label your food e.g. "Rachel's Gluten Free Brownies"
Little reminders help in big ways!