It is not a "bad" protein, some people are allergic to it and that makes it "bad" for them. People with Celiac Disease cannot eat anything with gluten because of the adverse effects on their intestines that makes it hard or sometimes impossible to properly digest food. Symptoms and sensitivity vary. Gluten is the protein that is used in breads mainly and in other foods to help with texture, cohesiveness, and elasticity.
Gluten is a protein compound that is found in foods that are made from wheat and other grains like barley, malt, or rye. A gluten-free diet has harmless amount of gluten that may cause an allergic reaction. Some people are unable to digest gluten and need to avoiding foods that have any of it. They have to read a lot of food labels because wheat is used in a lot of products as a thickener.