Caesar was able to conquer Gaul the good old Roman way- divide et impera- divide and conquer. The Romans and the Greeks before them had made inroads into Gallic society by delivering one thing they wanted: wine. With this and other material niceties the Romans were able to woo Gallic chieftains. The Romans would not have gotten far had they not had Gallic allies all over the place.
Even as Caesar was marauding about in so-called Long Haired Gaul, Gallia Cometa, he had Gallic scouts and other “embedded intelligence assets” all over giving him the low-down on Gallic fighting styles, strengths, hidden caches of food (a specialty of Celtic technology) and the political situation at each village