Oats don't contain gluten. Oats are often manufactured on shared equipment but the oat itself doesn't contain gluten. So the company was right in telling you their product is gluten free. You might have a reaction to oats but it isn't from any gluten "in" oats since there is no gluten in oats.
While not EVERYONE reacts to oats a lot of us do and most doctors and reputable celiac websites will tell you to stay away from them unless they are 100% gluten free grown and processed. Because of this I say it is absolute BS for a product to be labeled gluten free when it contains something known to make many celiacs react. I've heard of celiacs that don't react to a certain wheat product but do to others - that doesn't mean its safe and that doesn't mean something with wheat should be labeled gluten free! Besides that fact, it also contains barley so the point is almost moot anyway.