Short answers, pulled from the story.
The Gauls sacked Rome circa 390 BC under the chieftain Brennus, who led the Senones tribe. They defeated a Roman army at the Battle of the Allia and then entered Rome without further resistance, plundering the city before departing with their booty.
The principal Gallic peoples named by Greek and Roman writers as Rome's antagonists were the Senones, Insubres, Boii, and Gaesatae. The Salluvii, Vocontii, Allobroges, Arverni, and Aquitani also feature in later phases of the conflicts.
At the Battle of Sentinum in 295 BC, Rome faced a coalition of Samnites, Gauls, Etruscans, and Umbrians. The consul Publius Decius Mus performed the ritual of devotio, sacrificing himself in battle, and Rome won a decisive victory under him and his co-consul Quintus Fabius Maximus Rullianus.
Caesar's campaigns from 58 to 50 BC resulted in near-complete Roman control of the territory between the Atlantic coast and the Rhine. Caesar documented the campaigns in detail and, upon discovering some Gauls were receiving British support, launched the first Roman military expedition to Britain.
Cisalpine Gaul, meaning "Gaul this side of the Alps," was the name for the region immediately south of the Alps where Gallic tribes had settled. It was the principal theater of the early Roman-Gallic conflicts, and Rome systematically absorbed it between 223 and 193 BC, taking cities including Mediolanum, Bononia, Placentia, and Mutina.
After Roman forces subjugated the Boii in Italy, the geographer Strabo recorded that many surviving Boii retreated north across the Alps into the region later called Boihaemum. Strabo noted the territory was likely already inhabited by Boii before their defeat in Italy.