I gave you the battles of Hannibalic wars, which were well-documented, and in all the battles where Gauls were a significant force, they wavered, even when Hannibal was winning, such as Metaurus. Then we have the Cisalpine Gallo-Roman wars, or the Gallic invasion of Greece, where in most documented battles, the Gauls routed prematurely.
In general, the Gauls did not have the same drive to win, unless they were defending their homeland, as in the later stages of the of the wars for their homelands. And those skirmishes were not even documented for the most part, save for Caesar's commentaries. But by that time, the Gauls have changed radically, into a much more cohesive, centralised states.
Nor did their culture place great value on the collective victory - you can be recklessly brave in the beginning of the battle, earn some standing, and then retreat with everyone else, only gaining. But what is there to gain by fighting to death, especially when fighting in faraway lands in battles that do not have much direct effect you? As a Gaul, you do not have the same sense of national pride. you do not have a state and government to answer to. You have your hometown or village, and if needed, you will fight to death to defend it. Otherwise, why bother to fight, save for loot and honour?
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