The bananas we all know and can find in all local markets are actually called Cavendish Bananas and it's the most common type of bananas. Gerbils can feed on them and if given occasionally to complement their primary diet it does not present any danger. Usually, the very tip of the banana is enough for an adult gerbil and he'll enjoy it for a while just make sure they don't keep it somewhere in their tank since gerbils love to hoard food.