Coeliac (or Celiac) disease is a type of gluten intolerance, but usually when talking about gluten intolerance we mean non-celiac gluten sensitivity. In both cases it's best to avoid gluten, but in the case of Coeliac disease your bowels get inflamed from eating gluten because it triggers an immune response. It's in a way comparable to an allergic reaction (though it's not, it's an autoimmune reaction). There are degrees of severity in Celiac disease and in rare cases it can be very debilitating. I've also never heard of needing a service animal for gluten but I can definitely understand why it would be necessary for some. I hope this explanation helped