I don't eat meat either, and can't eat anything that has even TOUCHED beef, pork, or turkey. My mother in law puts turkey broth in EVERYTHING-the stuffing, the green beans, the potatoes...so the first year I ate salad. It was awful. The next year I offered to be in charge of a couple things-so I made the potatoes, beans, and brought a cheese plate and a dessert. That still kept with her menu, but made sure there was something I could eat.
I would definitely call and say something like "I would love to bring something, so please let me know how I can help. I'm not sure if you know this, but I have to stick to a gluten-free diet for health reasons-I don't want to be a burden with this restriction, so let me know what I can make and bring for the group, and we can all share without Grandma having to alter her menu." That way, it sounds like you WANT to help, and that you don't want to cause problems for Grandma