Gaul (Latin: Gallia) was a region of Western Europe inhabited by Celtic tribes—not part of Italy. The confusion may stem from Roman history, where "Cisalpine Gaul" (Gallia Cisalpina) referred to the Po Valley in northern Italy, settled by Gauls who crossed the Alps centuries earlier.
Core territory: modern France, Belgium, Luxembourg, Switzerland, western Germany, northern Italy (Cisalpine), and the Netherlands. Julius Caesar's Commentarii de Bello Gallico (58–50 BCE) chronicles its conquest. Major tribes: Arverni, Aedui, Belgae, Helvetii.
"Gaul" usually means Transalpine Gaul (France+). Cisalpine Gaul was a specific, early-Romanized sub-region. Modern Italy contains ruins of Gaulish settlements (Turin, Milan, Brescia), but Gaul as a historical entity lay largely beyond the Alps.