I am astounded. It was a brilliant move for Rome. Conquest usually is, and if we seem to be of the opinion these days that conquest is usually more trouble than it is worth, then this sentiment only gains such wide currency because we in the modern world have forgotten how to do it properly.
Frankly, the question is as puzzling as being asked whether America would have been richer, had it not slaughtered the American Indians and gained all the territories beyond the Appalachians; or whether Islam would have spread better, had Umar and the various Ummayyads played nice with their neighbors instead of marching armies from Nahavand to Poitiers; or whether....
But anyway, let's return to the Gallic War. How did it precipitate the Germanic invasions? The Germans were war-like peoples who were bent on expansion. The Rhine is actually a very nice barrier to invasions and much more defensible than the earlier position of the Roman border, in which Narbonensis lies beyond the defensible Alps and the Spanish holdings have no solid land route connecting them to Rome. Considering that the Germans were already invading the Gauls from across the Rhine, and that they would continue to press severely upon the Rhine border over the next four centuries even despite its defensible qualities, how long would an unconsolidated Roman empire with a long border strung out across southern Gaul from the Alps to the Pyrenees have lasted whilst Ariovistus and his successors were playing footsy with it?
(Btw, I like Primus Pilus' reductio ad absurdum argument that Rome would have been best to remain under the dominance of the Tarquin kings.)