As jake pointed out early in this thread, coeliac disease is a nasty thing to have and is fortunately rare. Not rare is digestive upset following consumption of food containing gluten; typically bread. Gluten is a protein responsible for giving bread dough elasticity and trapping the bubbles of "carbon pollution" that give leavened breads their texture.
The period of allowing the yeast to do its work in the dough is called proving. Modern breads use accelerants to reduce proving to its shortest possible duration, two hours or less. Traditional bread makers like Mrs Git allow bread dough to prove for 24 hours. A friend in the USA who makes award-winning pizzas proves his dough for 48 hours. The longer proving improves the digestibility of the baked product.
I suspect that many who think they are sensitive to gluten are reacting to crappy modern bread.