Better yet: don’t mark things with what they don’t contain, only things they do. Gluten free, is dumb, contains gluten is much better. What it doesn’t contain is nearly infinite, what it does or might contain is a much smaller list.
I don't think it's dumb, it makes sense to label something that would otherwise intuitively have gluten (like bread) with a gluten free label. It also lets someone who depends on it not containing gluten (or otherwise suffer potentially serious consequences) be confident that someone has checked over the whole manufacturing process to confirm it.
