Brennus
Brennus was chieftain of the Senones tribe who defeated the Romans at the Battle of Allia in 390 BC. He led an army of Cisalpine Gauls in the attack on Rome and held the city for several months. Brennus's sack of Rome was the only time in 800 years the city was occupied by a non-Roman army before the fall of the city to the Visigoths in 410 AD.
Vercingetorix
Vercingetorix (c. 82 BC - 46 BC) was the son of Celtillus a leader of the Arverni tribe.
He becomes king and chieftain of the tribe in 52 BC. He united the Gauls in a failed revolt against Roman forces during the last phase of Julius Caesar's Gallic Wars. Despite having willingly surrendered to Caesar, he was executed in Rome.