The idea of the Gauls and other "Barbarians" fighting in a chaothic mess doesn't make that much of a sense to me.
For starters, that would have meant a huuuuuge loss of human lives, even if you were victorious. You also find cultures less "civilized" like the Zulus that were fighting in complex formations, so it would be weird that the Gauls were an exception.
Who says the Gauls fought "in a chaotic mess?" Certainly not Caesar, who's just about our only source for Gallic military practice. Caesar continuously notes the use of tight formations among the Gauls--the Helvetii are said by him to have formed a phalanx, which allowed his troops to use their javelins to rivet the Gauls' shields together--and praises their speed of movement, their use of light-armed troops and cavalry, etc.