A Celtic Gaul can go in all sorts of directions longterm honestly.
A good POD would be Hannibal defeating Rome in the 2nd Punic War, but you're looking for something a bit later. I guess a more successful Cimbri migration into Italy that completely dismantles the Roman heartland and a more destructive Social War might do the trick. Perhaps no Marius would be a more specific POD?
So, Gaul by the last century BC was already rather Romanized. Narbonensis had been under Roman control since the 120s BC, but if Rome crumbles in the 00s BC, which is doable, they went through some tough shit in this time between the Cimbri, the Social War, and Mithradates, you could see Narbonensis return to native rulers.
By this point, the Arverni power has been largely stunted, but as seen OTL in Vercingetorix's rebellion, that isn't to say they can't make a comeback. The Gauls were a very disunified people, much like Italy pre-Rome in a lot of ways, with a patchwork of tribal confederacies and hegemonies across the region. The strongest amongst these at the time of our POD were the Aedui, Sequani, Pictones, Remi, Ebrones, Veneti, and Bituriges, as well as the Arverni and Volcae (who were both greatly weakened by Rome's presence int he South). Helveti and Boii were migrating into the region from what is now Switzerland and Southern Germany, and that could shake things up a bit. Not to mention the Germanic Suebi who were taking advantage of the instability. The Aedui and Sequani (the two most powerful tribes at the time) fell to them pretty quickly.
Given this hodgepodge of contenders, we'd likely see some amount of centralization amongst them around a hypothetical year A.D. 1. No unified Gaul, but probably larger political entities than before.
If Gaul is left alone from Italian expansion (which at this time is, admittedly, a tall order, though not impossible), I think centralization in the region is inevitable. Outside pressures demand it, not to mention the already pervasive influence from Rome, wether Rome crumbles or not. Republic-like oligarchies were becoming very popular amongst the tribal confederacies, not to mention the increased trade from the Mediterranean. Gaul will continue to export slaves and import wine and olive oil. If one tribal confederacy can manage to reform its military to something more professional (dare I say, more Roman?), then you've got something terrible to behold indeed. Gallic armies were massive, even by conservative estimates, and if such hordes were ever organized and made professional, there's quite a bit of potential there. My bet is on the Arverni, but I wouldn't count out any of the others immediately.
But, let's go with the obvious and frankly favorite, the Arverni, and see what could happen with them. The Arverni would rebound probably around the same time they did OTL, maybe a little later. If Rome is unable to hold onto its territories in Gaul, it will be the Arverni and the Volcae who swoop in to assert dominance in the region. If an Arverni general, perhaps Celticus or Vercingetorix, is ambitious enough, you might see the Arverni taking Massilia and Narbo and establishing dominance over the lower Rhone. This would be a huge economic boost in more than one way: not only would Massilia and Narbo hand over very wealthy ports to the Arverni, but also it would solidify their hegemony over the Loire-Rhone trade route, which was at the time the best route for goods to be sent to and from the Atlantic to the Mediterranean. If the Arverni can vassalize the Volcae, or even absorb them entirely, then they have no real native rivals in the South and can turn their focus northward.
Assuming migrations happen the same as OTL, the Helveti will probably be settled somewhere on the Atlantic coast in Aquitaine and will be made vassals of the Arverni. The Suebi will have actually done the Arverni a huge favor by subduing the Aedui and Sequani (the latter was an ally, but still) by eliminating two major rivals. I don't foresee the Suebi being able to expand much farther, and might even be subdued in turn by the Arverni after a decade or so to establish themselves in the region. Over time, the Suebi will assimilate into Celtic culture even more than OTL (Ariovistus was a Gallic name).
Arverni expansion in Gaul will, actually, look a lot like Roman expansion in Italy. It will take a few centuries to completely unify the region, and there will probably be a great deal of turmoil, but once the momentum gets going, it would be pretty hard to reverse. An Arverni-led Gaul would probably have borders that stretch from the Pyrenees, to the Alps, up the Rhine, and then just past the Sienne. Eventually Belgica might be conquered.
The pressing issues for Gaul would be warding off Germanic tribes (who would continue to see Celtic culture as the premier culture, not Roman as they shifted OTL after Rome took over Gaul. Evidence can be seen of this in contemporary Germanic material culture and the names of their leaders that all look suspiciously Celtic), and keeping the Italians at bay, probably promoting soft-power in that region to keep Italy unstable and disunified. Hispania, who might be a Roman rump-state, would probably be a threat, but given the sharp lack of empires that straddle the Pyrenees, I think neither side would be able to really do any lasting damage to the other.
Gallic language would probably look something like Breton or Welsh, with perhaps even more influence from Latin. Speaking of, the southern British Kingdoms would likely swear fealty or vassalage to the Gauls. I can imagine the rare army sent across the channel, but I don't think Gaul would ever conquer the island. This, of course, butterflies away Anglo-Saxon invasions 4 centuries later. So no talking about the English in this thread. They won't exist.
Across the Rhine, it's hard to say if Germanic pressures will continue to absorb the Celtic tribes there. At this point, that certainly seems to be where the momentum is heading, but if a stable and mostly unified Gaul can stall or even reverse it is something I'm not sure of. But the relationship between the two would certainly be a nuanced one. Germanic raids into Gaul would be common, and if Gaul is strong enough, they might exert their power or even invade in order to subjugate the violent tribes there to protect their heartland.
Art will probably be greatly influenced by Greco-Roman culture, though it will have a distinctly Gallic flavor to it. Perhaps Hellenistic sculptors will be imported as a way to promote prestige for nobility. In fact, prestige was such a pressing concern for Gallic elite, you might see quite a lot of impressive artistic and architectural endeavors over time. What they would look like? Probably more Greco-Roman than we'd like to admit, but still quite alien I should think.
I think with this POD, you can insure polytheism in the region for a while. Maybe some Jewish immigration? But the Great Jewish revolt would probably be butterflied away, so there's no telling really.
Military garb will look quite Roman as well, actually. By the time of our POD, what we imagine as classical Celtic armor and weapons were quite out of fashion in Gaul. But, surprisingly, it was the Romans who adopted Gallic innovations in this category, not the other way around. For example, the Gallic Helmet that would later be used by the Romans, as well as chain-mail, and gladius-like swords.