Goldsworthy may be exaggerating for dramatic effect here. There had been extensive trade networks between Gaul (and central / temperate Europe more generally) and the Mediterranean since at least the Hallstatt period (~800-500 BC) as the inhabitants of those regions sought prestige goods such as wine, jewelry and pottery in exchange for slaves, salt and other raw resources. Gauls traded for Mediterranean goods as far as Greece and Etruria and definitely Rome; in fact the notion that Gauls paid a single slave for an amphorae of wine is commonly cited among Roman authors as proof that the Gauls were a stupid and easily duped people, although that amphorae of wine was worth way more in the Gallic prestige system than a single slave! High status individuals collected retinues of followers on the basis of prestige goods that they could distribute at feasts and gatherings, so an amphora of Roman, Greek or Etruscan wine would have been well worth that price from the perspective of a Gallic warrior-aristocrat. Besides this previous knowledge of trade routes in Gaul, Caesar conducted his campaign with numerous Gallic allies who would have furnished him with information about the lay of the land. This is something that a proper classicist could tell you about, but I'm just here to inform you that Gaul was definitely not some barbarian wasteland unknown to Mediterranean folks.