Gluten is the protein in wheat that gives bread dough its strength and elasticity, allowing it to form air pockets and rise. Whole grain flours contain less gluten than refined white flour, which is why whole grain breads tend to be a lot heavier and denser. To compensate for this, bakers sometimes add additional gluten to whole wheat bread doughs. White bread that doesn't list gluten in the ingredients may have more gluten than whole wheat bread that does. But, as I said, unless you have a gluten sensitivity, this is irrelevant. See also: Gluten-Free Diets