I've what I find is an inconsistent image of Gauls literacy before roman conquest. On the one hand, my mental image is that they didn't leave much if any written traces. On the other hand, they formed a complex society and were already in tight contact and had lot of commercial exchange with Romans and Greeks, who for sure were literate and thus they were exposed to writing.

What are my misconceptions? If applicable, what are the major explanations for the fact that the practice of writing didn't percolate more in the Gauls culture?