Gaul was the Roman name for a territory comprising modern day France, Netherlands and Belgium as well as parts of northern Italy, western Switzerland, and western Germany. First subjugated by the Roman Empire under Julius Caesar, they were a part of the Empire for most of its existence.
The Gauls, who called this area home, were a Celtic people. There are still some elements of Celtic influence in places like Brittany, but their culture was largely overrun by the Romans and the Franks.