I am recently diagnosed, and have struggled with similar emotional responses from myself and from other people around me.
My last serious gluten exposure found me unconscious on the bathroom floor, then in an ambulance, and I spent 3 days in the cardiac ward of the hospital. My heart was affected by my exposure. While I may be much older than some who are newly diagnosed, I consider 60 to be pretty young by today's standards.
I am now hard-core about being gluten free and watching every morsel of food that goes in my body. Religiously reading every label on every pre-packaged food I consume. I am learning (LEARNING is the key word) to say "I'm sorry if my disease offends you, however I prefer to be alive and not in the hospital due to eating gluten. I'd love to come to your house, however, I will have to bring my own food with me. I don't want to offend you, and I want to spend time with you. If this is a problem for you, I understand."
And then I move on. It's hard. And as others have said, you find out who your true friends are.
When I go out and think I may be hungry, I take things with me. Little baggies of nuts. Heck, even those fresh fruit squeeze pouches of fruits and veggies come in handy if I'm hungry. They don't require refrigeration, take up little space and aren't heavy. A Banana is easy to tuck in my purse. A baggie with some of my favorite gluten-free crackers. Little packs of sliced pepperoni (gluten-free). A foil bag of salmon or tuna for a quick protein fix. A bag of potato chips that are from a gluten-free facility. I even picked up a special "food bag" to carry my snacks in if I know we're going to be out and about for a while.
It's extra work and requires extra planning. It makes me more conscious about what I am eating, why I am eating it and when I am eating it. I'm sorry if Grandma dismisses your feelings and doesn't respect your choices. But you are an adult and they are your choices.
Print out some info about Celiac and HD and give it to her to read at her leisure. Educate her.
I prefer to remain alive. While your disease hasn't progressed to the point of being life threatening, if left untreated (by continuing to eat gluten) it can very well progress to the point that it is life threatening.
I have way too much to live for -- and food containing gluten could kill me (maybe not that instant, maybe not today, but eventually it will destroy my body) -- and none of it tastes good enough to risk my life.