Originally, yes. They were "Gauls," as the Romans called them, who in the middle of the 1st century BCE were living in the area roughly from the Rhine to Normandy and north into what is now the southern part of the Netherlands. Already by the time Caesar added this region to the Roman Empire, the peoples there were somewhat culturally distinct from the Gauls/Celts to their South. They were a mix of Celtic, some Germanic, and subsequently Roman influence. The Germanic tribes of Late Antiquity called them walha, which referenced their Latino-Celtic dialect.

It is a little difficult to find information on the origins of Wallonia, due mostly to the activities of the region in later history. There is a book A Cultural History of Wallonia edited by Bruno Demolin (2012) which has a brief but nice coverage of the origins.