Gluten is the storage protein found in grains of wheat. But other similar proteins are also called gluten in a general sense and are widely found in grains and in at least one fruit (this fruit happens to be Buckwheat, but Buckwheat is neither a wheat or a grass).
Glutens are a family of very useful, very cheap, and very tasty proteins. Because wheat gluten is so useful, cheap, inexpensive, cheap, tasty, and cheap, it can be found added to most packaged or restaurant foods.
The “gluten” from each type of glutinous seed is a little bit different and has its own name, but all are loosely called “gluten”. Likewise, tolerance to these different proteins varies between people. The point is that some glutens may be safe for some people to eat while others are not.
More people have health problems with wheat gluten and very similar glutens in rye and barely than with other types of gluten and more people have health problems with wheat gluten than with any other single staple foodstuff. Most other glutens are generally well tolerated (but not always).
The chemical problem in Celiac Disease and Gluten Intolerance is a specific fragment of wheat gluten, a family of sequences of amino acids called Gliadin. Very similar fragments that also cause injury are found in rye and barley (also in oats but that is a special case).