I would say both. Even though the Romans had conquered other more prosperous areas, the loot from Gaul was enough to cause an adjustment in the value of gold in Rome. Gaul was also to become an important source of slaves for the empire, and Gallic agricultural production wasn't too shabby. Gaul proved to be a valuable addition to the empire; they provided raw materials such as wood (deforestation was rampant in Italy and other more developed provinces), metal, and handmade goods. In later antiquity, Gaul served as a buffer between Italy and the migrating Germanic peoples. And of course as you say, the conquest of Gaul gave Caesar the wealth and political and military power to take over as he did.