Gallia Belgica (sometimes given as Belgica Prima) was a Roman province located in what is now the southern part of the Netherlands, Belgium, Luxembourg, northeastern France, and western Germany. The indigenous population of Gallia Belgica, the Belgae, consisted of a mixture of Celtic and Germanic tribes. According to Julius Caesar, the border between Gallia and Belgica was formed by the Marne and the Seine and that with Germania by the Rhine. The area is the historical heart of the Low Countries, a region corresponding roughly to all of modern Belgium; the Netherlands south of the Rhine; the French regions of Nord, Picardy, Upper Normandy, and Champagne-Ardenne; and the northern part of the German Rhineland (not including the Moselle).
Read more about Gallia Belgica: Roman Conquest, Julius Caesar's Commentary, Formation of Gallia Belgica, Under The Emperors, Fall of Gallia Belgica, Belgica As The Name of The Low Countries