"Barbarian" populations

[Chilly5]

well, the gauls were a warrior nation, everyone in the army . ..

[Ranika]

Actually only a small percentage of Gauls were actually part of the army; they were essentially the equivalent of modern upper-middle class. They were the part of society allowed to vote for the magistrates and kings (non-warriors could only elect local officials). Anyone not in the warrior class only fought for short periods as militias. Most Gauls were either professional tradesmen or farmers (both skilled and unskilled labourers), not soldiers. There's no possible way they could have existed as long as they did with a majority in the military; they would simply collapse under a lack of food, a lack of armaments for their army, a complete lack of settlement development (IE; there would have been no oppida), etc.