Gauls
In 58 BC Julius Caesar launched the Gallic Wars and had conquered the whole of Gaul by 51 BC. He noted that the Gauls (Celtae) were one of the three primary peoples in the area, along with the Aquitanians and the Belgae. Caesar's motivation for the invasion seems to have been his need for gold to pay off his debts and for a successful military expedition to boost his political career. The people of Gaul could provide him with both. So much gold was looted from Gaul that after the war the price of gold fell by as much as 20%. While they were militarily just as brave as the Romans, the internal division between the Gallic tribes guaranteed an easy victory for Caesar, and Vercingetorix's attempt to unite the Gauls against Roman invasion came too late. After the annexation of Gaul a mixed Gallo-Roman culture began to emerge.
Gauls
The Gauls emerged around the 5th century BC as the bearers of the La Tène culture north of the Alps (spread across the lands between the Seine, Middle Rhine and upper Elbe). By the 4th century BC, they spread over much of what is now France, Belgium, Spain, Portugal, Switzerland, Southern Germany, Austria, the Czech Republic and Slovakia by virtue of controlling the trade routes along the river systems of the Rhône, Seine, Rhine, and Danube, and they quickly expanded into Northern Italy, the Balkans, Transylvania and Galatia. Gaul was never united under a single ruler or government, but the Gallic tribes were 
capable of uniting their forces in large-scale military operations. They reached the peak of their power in the early 3rd century BC.
The rising Roman Republic after the end of the First Punic War increasingly put pressure on the Gallic sphere of influence; the Battle of Telamon of 225 BC heralded a gradual decline of Gallic power over the 2nd century, until the eventual conquest of Gaul in the Gallic Wars of the 50s BC. 
After this, Gaul became a province of the Roman Empire, and the Gauls were ethnically and culturally largely assimilated into Latin (Roman settlers) majority, losing their tribal identities by the end of the 1st century AD.