Gallic society was not by any means nomadic, except, as mentioned, for mercenary bands and bandits. Gauls DID build large cities, had huge numbers of slaves, and were described as oppulent because of their habit of 'displaying wealth'. They also did outfit large armies early in the period (the situation with Caesar's involvement is very convoluted and shouldn't be taken as the standard by which the earlier period of Gallic development should be seen). Gauls built large buildings, complex city infrastructure (maintained roads, sewage, water, planned out districts, etc.), particularly as the period went on. However, the earliest Celtic cities would predate the start of EU:Rome, and were built the same way for the most part. They used huge numbers of slaves to build complex, large defenses, dug channels, built massive earthworks, and manufactured huge amounts of arms and armor; there even seems to be some standardization of design (compare later Celtic helmets in northern Italy to their contemporaries from northern Gaul; they're identical in their basic design, the variation is more in decoration).
Even Gauls outside the cities could not be termed nomadic. Gallic nobility and ignoble aristocracy lived on permanent farming estates of various sizes depending on individual wealth, on which they raised a wide variety of crops and livestock, some of which the Gauls actually introduced to the regions they inhabited. The estates could vary from a single family farmstead up to entire villages and towns built around sustaining an estate, with plenty of slaves to do the field labor, as well as care for the aristocracy dwelling there.
Slaves were a large part of Celtic trade. The Greeks seem confused how the Gauls could sell them so cheaply, in exchange for what they saw as such little wine for so many slaves (apparently not grasping supply-and-demand; the Celts had huge numbers of slaves, but little wine compared to the Hellenes). They knew how to utilize slaves on a large scale.
By the Romans' own account, it is more by a matter of politics and prolonged warfare depleting soldiery that led to the conquest. Germanic incursion, war between the two main powers (the Aedui controlling the north, the Arverni then Sequani controlling the south) depleted the armies, so, the Aedui requested foreign intervention (from the Romans). Not all of the tribes loyal to their head magistrate agreed, and, then, weren't loyal anymore; they abandoned the Aedui, and their control of about half of Gaul collapsed due to individual tribes, lesser magistrates, sub-kings, druids, etc., not wanting Roman aide.
Also, in 280 (I think that's the start given?), Rome wouldn't be economically superior to the Gauls. The large two federations, and the kingdom of the Aquitanians, would both probably be substantially richer. The Gauls mined huge amounts of gold, silver, and other precious metals, produced large amounts of tradable goods like linen, art in the form of metalwork, weapons, armor, etc. Again, they were called opulent. They were veritably loaded.
Also mind, when Augustus became emperor, Rome still had dirt roads. The pre-Roman Gallic oppida Bibracte, Alesia, and Gergovia, all had had roads covered with wood plank and stone for sometime from what we find digging up the sites. Rome really wasn't that amazingly different from the Gauls in a lot of ways. They were different, but they weren't seperated by 'Gauls: Mud hut dwelling savages, Romans: Enormous, well-ordered cities'. Gauls created a lunar calendar that until the advent of the computer, we couldn't make one so accurate. They did live in cities. They had armies, we know, because their contemporaries tell us about them, and we find physical evidence of actual armies from the Celts that can't be summed as cheaply equipped warbands, as well as standards and horns, evidencing command structure. They could fight in tight phalanx formations, or in looser formations, could commit to complex manuevers, and had a system of merit and reward, which encouraged sometimes rash action. But, if we focus on that, we should also focus on descriptions of Romans jumping on Germanic warriors' shields to get at the men in the German shieldwall, or the foolish rash behavior of the praetor at Faesula.
They were, in the sense of the word, a civilized people if Romans are being used as the measure (though I'd rather Hellenes and Carthage be used more as a standard for 280, if there needs to be one), because they were developed as the Romans, but had developed in a different way (their knowledge of mathematics, for example, was more used for astronomical calculations it'd seem, but, they apparently must've had a pretty advanced knowledge based on the Coligny Calendar). It's not a matter of being PC, there were people in Europe who were undeveloped, and truly pretty barbaric, but Gauls wouldn't be among them in sheer sense of technical development.