Romans both did and did not lumped together populations we now consider as "Celtic" in Europe depending on their needs, mental outlook or understanding. While they called "Galli" as much people of Northern Italy than southern Gaul, northern Gaul, Germania, etc. they also made distinctions depending of their experiences (Caesar considering the Celts* essentially distinct from Belgians and Germans which sort of true on a political level, but significantly less so in every other aspect). So an outsider look alone doesn't informs us much about what happened in Gaul between the IVth century and the IInd century BCE, something we rely primarily on archeologic evidence and interpretation to sort it out.

Now, it is widely considered that Gaul of the IVth century BCE and Gaul of the Ist century BCE belongs to the same civilisational ensemble in general, and that local evolutions were a continuation of situation known trough archeology but as well Greeks authors (Greeks having contacted Gauls since the VIth century BCE) then Romans in the period that matter to us. Even since then, these contacts allowed the appearance of proto-agglomerations and confederations in the southern Gaulish shore (Elysices, "Ligures", probably a first Celtic denomination) and were called the "Hallsttatian princes" of the Rhone valley and beyond in Champagne or southern Germany. This Halstattian archeological horizon wasn't as much succeeded than gradually replaced by a LaTenian material culture that comprise late Iron Age Gaul in is quasi-entierety until the Roman conquest, from the Vth century to the Ist century BCE, altough not being limited to the region.

Continuity, however, does not imply the absence of changes and the region actually knew a lot of by the IVth century : the movement of peoples and raids that happened in Italy, Balkans and Anatolia didn't spare Gaul itself. Newcomers, most probably from modern southern Germany and Czechia, entered in northern Gaul both settling relatively deserted regions in what became Caesars' Belgica (at the exception of Remi's territory that kept more similarities with past situation) and beyond, causing more widespread warfare in the Seine valley, likely, possibly leading to the settlement of Volcae in southern Gaul (a name found as well in Bohemia and Balkans, along the Great Expeditions in the latter) and eventually in Britain, altough in scales and numbers hard to assess.

These newcomers essentially shared the same LaTenian material culture than a local population they quickly mixed, with material variations (often, but not only limited to specific tribal or popular differentiation in weaponry or jewellry style). Overall, these migrations were common in Iron Age Gaul, and can be mirrored with the migration of Eburones, Helvetii and other Cisrhenani in Gaul in the IInd and Ist century BCE; without the cultural make-up of it being radically changed as it was after the Roman conquest.

At least considering the (very) limited onomatologic list we have at disposal, names as Brennos/us, Segobriges, Senones, Bolgios, Acichorios and several tribal or place names associated with Galatians in Anatolia (Tectosagii, Drunemeton, etc.) it seems that Gauls and Celts present in Italy, Greece and Anatolia spoke the same or a very similar language than Gauls of the Ist century BCE even if they did not necessarily shared the same geographic or ethnic origin.
Linguistically, the difference between Celts and Belgians is difficult at best to make out : while theories argues in favour of a distinct Belgian language (either close to Gaulish or dominated by a Gallicized elite), these aren't widely accepted, the mainstream understanding being that linguistic evidence we can tie to Belgians in epigraphy, onomatology and toponimy can easily be explained trough Gaulish (altough ancient authors making a difference between Celts and Galatians often argues of a small difference of language that can't be identified due to the lack of sources, and we can't asses the changes in Gaulish language in the period, safe the almost total change from -p to -kw, as in equuos/epos). It's not to say that there were not differences between Gauls, and especially in Belgium that leaves an impression of important diversity within a shared Gaulish ensemble, but we don't always know what to make of it while ancient descriptions and archeology rather points the similarities in way-of-life, institutions, spirituality, etc.

While Caesar describe Belgians as radically different from Celts, he nevers explains how exactly and virtually all his descriptions can be compared (while not all can be applied) to what exist in Celtic Gaul. Overall, we're talking of a great cultural continuity, and if Livy's description of Gaulish peoples for the IVth century BCE is accurate, even finding same peoples (notably Bituriges) than attested for in the Ist century BCE.

On the other hand, migrations from Gaul would cease after the IIId century, with a certain reorganisation of the territory : it seems that at this point, a differenciation between Belgians and Celts was institutionalized into fluctuating and ill-defined regional ensembles gathered by a sense of specific kinship, genealogic proximity but also shared interests thus aside the Assembly of all-Gaul and the annual druidic assembly (to whom all Gaulish peoples were repsented, including Belgians) existed a specific "common assembly of Belgians", probably something similar for Armoricans (themselves considered as Belgians by Strabo on account of kinship, but considered Celtic by Caesar, maybe for their political proximity) which was neither exclusive to other relations (Aedui still having clients among Belgians) and neither seemingly set once and for all and rather including newcomers or neighbours depending on the situation.

The greatest changes, however, were due to foreign pressures, and especially the rise of Rome in the Mediterranean basin, both providing with the opportunity of continued mercenariship, but also effectively closing off migrations in the region, the Gaulish "Mediterranean tropism" being set by a relation with Romans as allies (notably with Aedui), trade partners (the establishmen of a silver coinage based on the value of Roman denarius common to several peoples) but as well a percieved ideological threat both by Druids, but also an important anti-Roman faction found in most peoples. This influence led to what had been described as "revolutionary changes".

The first, and obvious change, with the Gaulish landscape, would be the emergence and development of oppida, proto-urban fortified sites (some of them true cities, such as Avaricum inhabited by thousands according Caesar) which headed local trade, agriculture and manual craftsmanship and politics to the point some peoples as Aedui effectively went trough a political centralization by the time of the Roman conquest. What made a warband in the Ist century BCE was vastly different from what made one as Brennos' in the IVth : while Gauls were employed as skilled mercenaries and "drill instructors" by Carthaginians, Etrusceans and Greeks already by then, war got "democratized" and semi-professionalized, at least in Celtic Gaul by the large inclusion of clientelized free-men, following a charismatic and wealthy patron in war against rival peoples (less for territorial takeover and more to assert the, oligarchic, interests of the polity and patrons).

It's possible that Druidism lost its strong influence by the IInd and Ist centuries, due to both the exportation of a "Roman way-of-life" material culture and its immaterial habitus, the rise of aristocratic political autonomy, and the unability of Druids to deal with the consequences of Cimbric and Teutonic wars in their traditional role of diplomats and mediators. This precise point, however, is still debated from the idea that traditional Druidry was basically dead by the Gallic Wars (which is the thesis of Jean-Louis Brunaux) to the idea druids still had an important political and social role, including in Celtic Gaul.

The chariot disappeared from Gaulish military equipment during the period, except as a prestige device comparable to its use in Rome (an use possibly borrowed from Gauls) and replaced by a cavalry whose horses got smaller in size. Leather protections and armour seems to have been more present by the Ist century, alongside the mail coat, while helms (inspired by Roman helms,are rarer and less elaborate) and shields (wooden, but also made of wicker and bark) were more common and allowed defensive formations (probably close to testuda, and possibly having inspired it). Axes are essentially abandoned, a tendency already present in the IVth century, but spears are more specialized and with longer and thinner points, as swords got longer.

While the peoples were essentially the same, they we so in the sense 2020's French and Germans are the same than XVIIth century Frenchmen and Germans : there's certainly a linguistic, cultural, anthropologic and somewhat political continuity (and, critically, an acknowledged one) on societies that obviously changed a lot mentally and materially. Unfortunately, as we have to rely essentially on archeological sources, what changed and how isn't always clear.

*Celts being understood there as the inhabitants of Celtic Gaul, roughly the southern 2/3 of Gaul, while Belgians, or Galatians, and populations of southern Germania or Spain being Celtic according our modern common definition