This is rather outside my bailliwick, so I'm afraid I mostly have to cite secondary or tertiary literature.

My understanding is that current knowledge of pre-Roman Gaul is largely shaped by archeology, as supplemented by written sources. There are only a few Greek and Latin texts that provide anything resembling a useful first-hand account of the social and political organization of Gaul prior to and during the Roman conquest. Unfortunately, one of our most comprehensive contemproary sources on Gaulish political and religious life is Julius Caeser's own Commentarii de Bello Gallico, which is obviously colored by his own political agenda in writing it!

Furthermore, a later Roman historians or ethnographers like like Pliny or Tacitus often didn't actually visit the areas they wrote about, but seem to have copied other second or third-hand accounts.

So basically, our understanding of the society and culture of Pre-Roman Gaul is shaped largely by archeology and epigraphy rather than texts. The Gaulish peoples of what is today France and the low countries seem to have made only limited use of writing, via a variation on the Greek alphabet. We have only a few coins and inscriptions. If there were ever longer written works composed in Gaulish, they no longer exist.