Buckwheat is an excellent substitue for oat bran. But oat bran itself is also pretty safe. It doesn’t contain gluten by itself, but it is usually produced in a gluten environment. Even people who have coeliac can usually safely consume one cup a day.
Bobs Red Mill Oat Bran doesn’t contain gluten.
Whatever you do, just try it a little bit in the beginning and add more day by day.