Is gluten synonymous with wheat?

No. Gluten is a protein found in wheat- also found in rye and barley. Its not synonymous with wheat but is a component of wheat that is avoided in people with allergy to wheat or celiac disease- an autoimmune disease to gluten.
No. Gluten in the United States is mainly from wheat, because wheat is such a large part of the average american diet. However, gluten is also in barley, rye, and spelt. Malted barley, or malt, has gluten just like barley does.