While it is true that Caesar often oversimplifies the distribution of peoples and sometimes gets things wrong, he was not making it up. All Indo-European societies were tribal in their organization at one point or another, and those old structures are clearly visible even at Rome or Greece (both of whose citizens were organized in "tribes"). The Celts and the Germans were very clearly divided linguistically, and to a certain extent culturally. Some others were more blended, like the Boiī or the Treverī. The Celtic tribes as described by Caesar were fiercely independent, had their own internal governance or else pined after the days before they had lost it, and often imprinted their specific identities on place-names in their territories, many of which survive to this day. Bayeux, for instance, or Nantes, or, in Gallic northern Italia, Susa.