Southern Central Europe, roughly upper Danube river basin and neighboring regions, is hypothesized as the original area of the Celts (Proto-Celts), corresponding to the Hallstatt Culture. Some closely fit the concept of a tribe. Others are confederations or even unions of tribes.
Cisalpine Gaul (Gallia Cisalpina), also called Gallia Citerior or Gallia Togata, was the part of Italy continually inhabited by Celts since the 13th century BC. Conquered by the Roman Republic in the 220s BC, it was a Roman province from c. 81 BC until 42 BC, when it was merged into Roman Italy. Until that time, it was considered part of Gaul, precisely that part of Gaul on the "hither side of the Alps" (from the perspective of the Romans), as opposed to Transalpine Gaul ("on the far side of the Alps").
Transalpine Gaul, meaning literally "Gaul on the other side of the Alps" or "Gaul across the Alps", is approximately modern Belgium, France, Switzerland, Netherlands, and Western Germany. in what would become the Roman provinces of Gallia Narbonensis, Gallia Celtica (later Lugdunensis and Aquitania) and Gallia Belgica. Some closely fit the concept of a tribe. Others are confederations or even unions of tribes.
List of peoples of Gaul (with their capitals/major settlements):