I don't expect gowther to ever be a top combatant TBH. He has a powerful, almost broken ability but realistically speaking it works best when he can catch his enemies of guard and lacks the actual capacity to harm his enemies. As for his magic, isn't it the same as papa gowther's? To put this in context, I would argue gowther becoming a top combatant would be like melascula being considered a top combatant. Sure, there are circumstances in which either would win without effort against much stronger enemies (say, melascula eats your soul) but that is not quite the same as either being the strongest.