Gauls, especially in central and southern Gaul, had developed astonishingly sophisticated civilizations by the time of the Caesarian conquest due to their proximity to and participation in the Mediterranean economy. Mediterranean luxury goods provided by Etruria and later Massilia were central in the Gallic prestige economy, which was incredibly important for client-patron relations between the Gallic warrior-aristocracy and their subjects. In exchange, the Gauls provided raw materials and slaves, and furnished the Mediterranean states with a considerable amount of mercenary troops.

Now I'll give you a more direct description of Gallic society pre-conquest. It seems that by the time of the Caesarian conquest, primitive states had already developed independent of direct Mediterranean influence. Society was intensely stratified between the warrior-aristocracy, the druidic caste made up of members of that aristocracy, the "men of arts" such as craftsmen and a mass of peasantry who Caesar describes as being utterly dependent on the aristocracy. This may be exaggeration on the conqueror's part, however I believe that this intense hierarchy was a likely reality due to Gaul's relatively dense population and near-total reliance on sedentary agriculture. It would have been likely for the aristocracy to achieve political and social supremacy by controlling land and land distribution, something that the "Celtic" aristocracy in Ireland never really achieved until about the 16th century.

When Caesar enters Gaul, you have a stratified and hierarchical society, but its political structure is uncertain. Caesar describes Gallic polities as republics with elected magistrates. It may be likely that some Gallic states had cast away monarchical systems of political organization and developed republics led by the local aristocracy, much like Rome itself. On the other hand, it is possible that Caesar was describing non-republican Gallic political offices using republican language, either because of his misunderstanding of those institutions, or to simplify them for his intended audience.

Regardless, we know that complex political structures based on sophisticated social hierarchies were the norm in Gaul. But that's not all: urbanization appears to have begun shortly before the conquest. The locus of political power seems to have been divided between the rural homesteads of the aristocracy and the newly constructed oppida - fortified urban spaces with some permanent residents and craft production. These weren't full blown cities like Rome or Athens, and may have only been fully occupied during times of war, but nevertheless their widespread construction and increasing role in commerce and production indicates a trend towards urbanization that was cut short by Caesar.

As well, it seems that some sort of primitive bureaucracy was in the works. We know that the Gauls were literate and used the Greek alphabet, but druidic and historical lore was unfortunately confined to an oral tradition. Gallic participation in the Mediterranean economy also meant that the use of coinage was widespread, at least in central and southern Gaul. The Gauls actually minted their own coins before the Romans ever did, but coinage's early use was as a prestige good based solely on the intrinsic value of the coin's metal. The transition from coins as a prestige good to an actual form of currency implies increasing political authority in the centuries before Caesar because some sort of authority was required to ensure the value of coinage.

So Gallic society on the eve of Caesar's conquest was one that showed astonishing sophistication, hierarchy and political and social organization. Given the trend towards increased political authority, evolving forms of statehood, urbanization and bureaucracy, I believe that Gallic states of comparable sophistication to those of their Mediterranean neighbours would have certainly developed on their own, and within a short amount of time, had Caesar never crossed the Alps.