Although most of the time butter as well as margarine are in fact gluten-free, that is not always the case. If you are gluten intolerant, it is extremely important to check the labels on whatever you are eating. Butter as well as margarine are not made from grain which is where gluten comes from, though it is still important to check labels religiously because of the fact that some manufactured butter does have gluten in it.
Those who want gluten-free butter will want to choose the plain brand in order to avoid consuming any. This is especially important for those who have a gluten intolerance and will therefore have an adverse reaction if it is consumed. Organic butter is always a good option for those who do not want to ingest any gluten because they have some type of sensitivity to it.