In book seven of the De Bello Gallico however, Caesar makes Vercingetorix say that trying to beat the Romans in pitched battles not only was detrimental, but near impossible, and that the Gauls should employ a tactic of scorched earth around the enemy, while constantly harassing the Romans with cavalry, and that’s exactly what they attempted to do. Vercingetorix knew the Gauls wouldn’t stand a chance in battle, and so acknowledged Caesar himself. They certainly weren’t the Romans’ equals.Except they completely did. If you changed out the people/place names for Roman ones, there'd be almost nothing in Caesar's account to tell you he was fighting Gauls instead of Pompey; in terms of military skills, discipline, and organization, the Gauls were very much comparable to the Romans. Caesar uses pretty much the exact same specific military vocabulary to describe Gallic armies as he does Romans; he explicitly calls Gallic and even German formations phalanxes, which Romans only used for heavy infantry formations that displayed good order and discipline. Metalworking was very highly developed in Gaul, and the country quite wealthy, so we should expect armor to be widely available. Moreover, when troops carry large shields like those of the Gauls, torso armor is of comparatively marginal importance; even in periods where less armor is common, the typical pattern of wounds found on skeletons is a wound to the limbs [being much easier to reach without making one too vulnerable to the enemy], followed by a finishing blow to the head. With this in mind, it's not surprising that the quality or availability of armor pretty much never figures in Caesar's accounts of the conquest of Gaul. The fact that he accomplished what he did was absolutely extraordinary, and largely came down to his ability in diplomacy, strategy, and canny battlefield leadership, not the quality of troops.
Gaul might have been a rich country, but not so all Gauls, some tribes were certainly richer than others, and none had a professional army like the Romans. I can easily imagine some Celtic soldiers fighting with nothing more than their kitchen knives.
All ancient historians employ the terminology they’re more familair with to make their readers understand what they’re talking about, it doesn’t mean that Celtic armies employed a standard “phalanx” in their tactics. Their main MO was to charge against the enemy, rather than stand their ground, thus the ancient topos “they’re fierce soldiers, but they tire easily.”
Caesar was an extraordinary general, and what he accomplished in Gaul only proves it, but we should never underestimate the impact quality troops can have in a campaing. Scipio Emilianus couldn’t beat some Celtiberians because his men were of abysmal capability, Sertorius couldn’t destroy Pompeius in open battle, despite being a far better general, because what Roman troops he had were lost by his lieutenants and the Iberians he had weren’t up to that task, and Agrippa had a very tough time with the Celtiberians and the Asturians because his troops were heavily undisciplined and wouldn’t follow his orders.