Did the Roman's not have an advantage in that their army (post Marian reforms) was professionalized whereas from my understanding the Gaulish and German armies were made up of a small number of elite warrior-aristocrats, their divided professional followers and a majority of non-professionals called up for the campaign?
Surely they had an advantage, but the so-called "Marian reforms" (a term that I've never encountered used among Classicists, though I think military historians like Goldsworthy like to use it) were neither so ground-breaking as people seem to think nor were the differences in Gallic armies so profound to be that much of a difference. Gallic armies tended to be composed very similarly to Roman armies, with large bodies of heavy infantry, equipped in mail, good helmets, tower shields (like the Romans usually made of leather and wood), spears and swords. Most Gallic people also fielded excellent heavy cavalry, which Caesar even admits he could not make a dent in until he got hold of some German cavalrymen, who were aggressive enough to counterattack the Gauls. Mind you, the Gauls were not a single people, culturally or even linguistically. The Romans lumped all of "Gaul" together, but as Caesar points out the divisions between these related groups often made them no more similar than the Romans and the Greeks. The Helvetii are described as fighting in a densely-arrayed phalanx, the Nervii are described as preferring javelins and ambushes, and the Aquitani were impressive sailors. To generalize about the Gauls is like generalizing about all barbarians together.
Even so, the changes in the Roman army were not those of equipment or really tactics. Marius' legislation did away with the property requirement, that's about it, and most of the changes in the army did not occur until Augustus' lifetime. A big fuss is always made for some reason about Marius' abolition of the property requirement, and it's often misunderstood to mean that Rome's army suddenly became professional. It means nothing of the sort. Roman armies were not professional armies intended to remain at arms permanently until Augustus' reorganization of the army and the establishment of the Augustan frontier outposts, which had to be manned permanently. Sulla's establishment of promagisterial governorships is of vastly more importance to the establishment of the Roman military force than Marius' abolition of the property requirement. The establishment of promagistrates with proconsular or propraetorian power in the provinces meant that enormous military forces were present in each province for the five years of a general's term. But they were not standing forces. Governors were expected to disband their armies after their five years of service, and most, like Piso and Gabinius, did--exceptions like Pompey's veterans are, well, exceptional. Caesar's legions had never seen combat when they marched into Gaul--presumably there were veterans among them, but they almost certainly were mostly green troops, as his legions had been raised, like all promagisterial legions, specifically for his governorship. There isn't much of a difference between these forces and the Gallic troops. Simply because the Gauls were composed mainly of non-aristocrats doesn't mean anything--Henry V's army in France was almost all highly-trained peasants, and given what Caesar tells us of the Gallic armies we should not imagine that discipline was low and equipment poor simply because these were not professionals. The Greeks fielded some of the most disciplined troops in the Classical Period, so much so that they were famous for it, yet even cities like Sparta relied almost exclusively on levied non-professionals.
Caesar is highly praiseworthy of Gallic armies. He's always going on about their bravery, their inventiveness, their discipline, and (once Vercingetorix organizes them) the skill of their commanders. Gallic armies were able to perform rather impressive feats of complex maneuver, such as at Gergovia where they managed to outmaneuver Caesar. The actions of the Gallic force at Alesia are really extraordinarily impressive--they utilized everything from night attacks to relief armies, and several times almost broke through the Roman lines. These are not the actions of ill-disciplined, poorly-led mobs. A couple clear advantages can be pointed out, though. The Romans were fighting a divided enemy, up until Vercingetorix. That's not too much of an advantage, though, because Caesar's enemies pretty much constantly outnumbered him anyway, but it did mean Caesar wasn't attacked on all sides when he first marched into Gaul. Caesar was also a brilliant leader, and brought the same talent that he had showed in the political arena into battle, despite having no military experience as a commander worth speaking of. In particular the speed at which Caesar was able to move through Gaul was shocking, even by Napoleon's time--as I recall Napoleon claimed to have modeled his rapid advances on Caesar's blitz through Gaul. This no doubt has to do with the training of the legions--inexperienced though they were when they first entered Gaul they were highly trained, and though Gallic troops could be expected to be well-trained, they were not trained to the precise, standardized degree of Roman troops, that much is clear. And of course the Romans by the time of Vercingetorix's revolt were highly experienced--these same troops had marched through Gaul and knocked off individual states one by one, gaining huge amounts of combat experience from it
As for Caesar's numbers, they are no doubt inflated, but probably not that much--certainly not as much as many of his contemporaries. Livy's and Herodotus' numbers, for example, are notoriously untrustworthy. Still, the Gauls undoubtedly gathered large forces. Caesar was routinely outnumbered, that much is obvious, and the Gauls were really quite sophisticated, with rather well-built cities and very fertile ground that was harvested using methods no less behind those of the Romans--many states were certainly capable of fielding tens of thousands of troops. Consider the Athenians--during the Peloponnesian War Thucydides says that the Athenian army could field about 20,000 hoplites without having to rely on allied contingents. Athens is vastly smaller and much less fertile than most of the states of Gaul. By Vercingetorix's revolt the majority of the Gallic states had united together (alas, too late) and were collecting forces to oppose Caesar together.