I but I also don't think it's considerably more implausible than the OTL rise of Makedon.
Macedonia didn't began as an imperial Greek confederation : far from being backwater (rather under constant pressure) it was a model of the ethnic Greek states that co-existed with poleis since the archaic period. Meaning it structurated itself early on, even if for a while under the dominance of Persian Empire. It benefitted, furthermore,from the presence of a develloped shore which was in direct contact with the "worthwhile" Greece so to speak, but whom protection was significantly wkeaned by the crisis of the politeis in the IVth century. (A good argument could be made against the rise of Macedonia in the case of an Athenian clear victory against Sparta). So saying from "scratch" is a big exageration.
Meanwhile, Gallic states develloped in a roughly similar timeframe, and as Helladic states never really fancied the idea of a Pan-Gallic unity for all the unifying strutures existing (cultural, religious, linguistic). More over, the bigger Gallic hegemonies and archeis followed a mediterranean tropism more or less affirmed, meaning a North/South development (really roughly). It's more or less the case with Arverni, and obvious with Aedui. I they suddenly went in a conquering spree; which was rather against the political traditions : you mentioned the
brenns ("Brennus" and "Brennos" of Rome and Delphes), which is the title of warchiefs, not leaders of a settled people; then South it would be. From there, you might end up with a particularily strong complex chiefdom, but the sheer weight of Gallic political substructures (which were rather participating, than ignored, by the political constructions) would make an imperial drive rather hard, while it would be easier than unifying Gaul as a whole.
I'll of course gloss over the demographical and territorial difference between Gaul and Greece, as it's doubtlessly obvious to you.
Let's be real, if someone wrote that as a TL, it would be denounced as ASB, space-filling empire to the extreme.
"There's people that couldn't for their life understood how hegemonic empires can rise quickly" isn't the same than "It's ASB but in real life",tough. To be honest, I find this to be a really weak argument, in spite of how many people repeat it.
I can at least imagine the Arverni subjugating their immediate neighbors, take control of the Rhone
As the events of the IInd century BCE points, it was hard for an hegemonic power to appear without banding everyone in sight against it. Arverni trying to take against everyone in the region (which, must I point, where were most of the strongest peoples and archê, such as Aedui, Allobroges, Vocontii, etc.) would be difficult.Rome managed to pull it because it was...well, Rome complete with comically disproportionned resources and military.
The social structures of Gaul basically made hard to gather a large army, not only their nature as aristocratic states, but as well cultural features such as Druidism, as Druids more or less played the role that the Church did in the Xth century and the "Peace of God" meaning regulating warfare and finger-pointing whoever was a dick. While warfare was relatively current in Gaul (altough not endemic), it was closer to a political strong-arming competition than annexionist.
if they make contact with a dissident political faction in Massalia, the gates might be opened to them
This is particularily not going to happen : if anything Massaliotes could agree on, it was that the balance that kept Celto-Ligurians at bay was precarious. Massalia as an archê already lost significant points, such as Arles (Theliné) in the IIIrd century, and their partners in the Gulf of Lion (such as Elysices) were disappearing. I think it's fair to say there was a strong political-identitarian point there, and ancient authors make pretty clear that Massaliotes were wary of their neighbours (Silius Italicus, notably)
To be honest, such turns of events looks like a RTW game (I say this without mockery, I did a game that was suspiciously similar) but makes little sense in the IInd century BCE.
