So we keep Caesar out of Gaul. The Germanic tribes were moving across the Rhine and into Gaul, and at some point there is probably going to be some kind of tipping point invasion, where the tribes of central Gaul feel quite threatened by the Germans. The Romans are distracted by some political infighting, and so the Gauls are on their own. They create a tribal alliance to fight off the Germans, and elect one of the tribal leaders as the "High King" or something of Gaul. The "High King" fights off the Germans and takes steps to make his title stick once the Germans are finished. The other tribal leaders don't like this, but by the time the Germans are defeated the Romans have settled their differences. The Romans see the "High King" as a low cost alternative to having to conquer the Gauls and defend Gaul against the Germans, so they support him. With Roman support the "High King" makes his title stick, and the Kingdom of Gaul, a Roman client kingdom, is born.
Gaul was a nice piece of real estate, but I think that the Romans would be just as happy letting it be semi-independent and not having to spend time and money administering it. With a Roman-inspired and trained army the Kingdom of Gaul manages to push the Germans back over the Rhine and expand into Germania. They develop their own unique military culture, which puts a much greater emphasis on permanently fortified towns as the cornerstone of territorial defense. This reliance on fortification serves the Gauls well, as they push into Germania via the acquisition and fortification of trading towns. Gaul develops a kind of pseudo-feudalism that is an out-growth of its tribal origins. The difference between the civil and military is never really clear, and Gaul remains a heavily militarized society, with a small military elite supported by a vast number of peasants.
With an independent Gaul that maybe does some expansion on its own, I think that its possible that they could develop the V-shaped iron plow. In OTL I believe Europe got the V-plow from the Chinese, though it would have been really useful in Europe, and could just as easily have been invented in Europe. If it is, then Gaul will get a big population boost, which will do all kinds of interesting things.