We tend to think that the Romans viewed the Celts as barbarians. In reality, the Celts were very integrated into the Roman system. Celto-Iberians had fought in the armies of Hannibal during the 2nd Punic War, and they were very well known in later Roman armies. (There is a nod to this in referring to Russel Crowe’s character in Gladiator as “The Spaniard”. In those days, the native inhabitants of the Iberian peninsula were ethnic Celts who had become Romanized.)
The Celts of mainland Europe very much embraced the Roman way of thinking, but they were ultimately displaced by the German tribes during the Great Migrations and the Hun invasions.
Galatia was a unique region of Asia Minor (modern Turkey). It was populated by Celts, the Iron Age inhabitants of western Europe. This particular group of Celts (called hellenogalatai or “The Gauls of Greece”) had first arrived there as mercenaries in the 3rd century BCE and stayed as a ruling class. As late as the time of Jerome in the 5th century CE, the Galatians still spoke their own Celtic language.
It is fascinating to think just how easily the Romans adapted their system of government to whatever group they encountered. While Julius Caesar was fighting the Celts in Gaul in the early 1st century BCE, their cousins had beaten the Romans to Asia Minor.