Lomg swords were effective too, do not think that Rome won all battles and certainly do not think that Celtic gear was not efficient, as far as I believe, the question here is that Romans chosed the best gear for their close quarters, close combat and celtics choose their best gear fo their tatics too try to imagine that you have an scutum and some two meters tall, heavy muscled Gaul is attacking your shield with a long sword and all its weight... you can see that the Gladius Scutum combination may have had a hard time blocking, avoiding and counter attacking this... still, victories were not only Romans and can not be credited only to combat gears