Cæsar is certainly not the ideal source, and he does carry with him many cultural assumptions, but I would not certainly consider his report to be “very ignorant and unreflected.” Not only does he present the Gallic people as a worthy enemy, but he actually gives us valuable information on their political and religious organisation. He fares far better, for instance, than Cicero. Of course, if what you expected was information on the day-to-day life in pre-Roman Gauls, well, it is not very surprising that a campaign report omits these details. You may also be interested in Strabo's (a Greek) Geography, but you will find the same stereotypes (“[Gauls are] war-mad, and both high-spirited and quick for battle, although otherwise simple and not ill-mannered”), but also information you may find more “meaty”. Diodorus of Sicily, book V also has interesting stuff in it. We happen to have lost the most complete work on the Celts, by Posidonius (another Greek writer); however, his ethnographic digressions were used as sources by other writers, and we also happen to have retained a few interesting fragments.

Otherwise, as usual with most “barbarians,” there is no independant source; then the work of the historian is to sort out the information of the texts. The closest thing we have to texts from a non-Greco-Roman perspective may be Greek inscriptions made by Celts. Archaeological research, however, has provided us with very interesting information on the urban culture of pre-Roman Gaul, and with its high levels of trade, which dispells the myth of the forest-laden Gaul, and bring to light a new perspective on several elements (among which their religion).

It's also important to remember the archaeological evidence available that can give a 'real' indication of how the Gauls were living before the Roman invasions. It's only when we combine the written evidence with the physical remains that we get a proper idea of what was happening (and even then it can still be patchy!)

Part of Caesar's issue is that although he was writing an account in part to give an accurate reflection of what he was encountering and the peoples he interacted with, the Gallic Wars were also being sent back to Rome to be read by the Senate and educated classes, so some hyperbole is likely to have slipped in somewhere! Similarly, the archaeological sources are limited in the amount that was preserved and how the archaeologists dealing with the material choose to interpret it.

There is a plethora of academic material dealing with just this sort of interface between the Romans and native societies (particularly in the west). There is a really good book by David Mattingly which goes over the Roman impact in Britain (which also briefly featured in Caesar's Gallic Wars) explaining the Roman influence on the island right through the Roman occupation.

The advantage of archaeology is that you can also get an idea of how the society functioned before Rome even started expanding into Italy. It's important to appreciate that for the Gauls etc, they may have had a very different idea of 'identity' and occupation that the Romans. When these modes of thinking came into contact with one another, there was lots of space for misunderstanding and cultural confusion.

Diodorus Siculus and Tacitus (Agricola for Britain & Germania for Germany) Also give some more indication of the people who lived in the areas of Roman interaction.