Oats are naturally gluten-free, however, most of the oats available in stores are processed in facilities that also handle wheat. If you're avoiding gluten for dietary reasons, these will probably be fine. However, if you or someone you're making these muffins for has a severe gluten intolerance or allergy, be sure the oats you are using are certified gluten-free.
Just like with banana bread, the bananas you use for these muffins should be super ripe and brown-speckled. I like to save over-ripe bananas in a zip-top baggie in the freezer (just peel them, then freeze), this way I always have them on hand. Frozen bananas will be very dark and slimy once thawed (appetizing, I know), but don't worry, they're perfect for baking. (Bonus tip: if you don't have any overripe bananas, you can take underripe bananas and bake them on a baking sheet (peel on) at 350 degrees F. for 5-6 minutes, or until the skin has turned black. Scoop out the insides, and you're good to go!)
*I recently made a batch of these and totally forgot to grind the chia seeds... and they turned out perfectly anyway. Feel free to skip the grinding and use whole chia seeds instead. (If you'd like to use flax seeds, I still recommend grinding them since they're larger and more noticeable in flavor and texture.)