Please, don't explain (alleged?) military superiority of Gallia with the 'argument from isolation'. It is even harder to swallow than no explanation. (Your mileage may vary).
Another thought of the Gallia invasion: it is unlike any of the previous wars. Most (all?) of them had some, how to say it, humble beginnings. Small-scale conflict between mining groups. Destruction of some facility. Such things. It is not about realism, it is about story and gradually increasing tension and scale of the conflict. Gallia's invasion begins with already high stakes. Very unusual.