You're right on Caesar making a difference between Gauls and their neighbors on depicting the latter (including Britons) as "more barbarous" that Gauls. Nowadays, we can point to the archeological discoveries made since decades to argue that Gauls did have a structurally developed civilization, having made their land an agricultural powerhouse trading with the ancient Mediterranean civilizations (Etruscans, Phoenicians, Greeks and Romans) centuries before Caesar, having reached a high level of craftsmanship technicality, and that they developed sophisticated political and religious life. On the other hand, both Britain and Germania does seems to be less connected overall with what was the heartland of Europe, the ancient Mediterranean basin.

Were there that different tough? It's important to remember that "Gaul" and "German" are Roman-made categories, which belong to their ethnographic and mental conception : Caesar's De Bello Gallico weren't made to provide an ethnographic study for the sake of it, but were originally rapports of campaign sent and read to the Roman Senate and the Roman people, and as such mainly focus on which political and military actions the general did and which political and military situations he had to deal with. He does describes Gaulish, British or Germanic societies as a whole from time to time, but that's mostly to both give a context on which peoples he's submitting (with the subtext that, because Gauls are "less barbarians", they are a fine addition to Rome, contrary to "more barbarous" Britons and Germans).

Being aware of this, we can't simply dismiss Caesar's commentaries out of hand tough : it remains the first ancient source giving a somewhat extensive description of Britons and Germans; and while the general can be biased or silent on key issues, he was not making things up as he went, several of his descriptions or affirmations being verified archeologically, especially as Romans already knew about Gauls trough their wars with them in southern Gaul, and the very extensive trade and diplomatic relations they had with some of their peoples (for instance, Cicero hosted Diviciacos, an Aedun druid and ambassador, not only because of the alliance between Aedui and Romans, but as well because Cicero had important interests in the wine trade in Gaul and especially with Aedui).

Poseidonios of Apameia, the universal man of the ancient Mediterranean basin at the turn of the millennium, went in southern Gaul (then already taken over by Romans) in -80, and his ethnographic work served as basis for virtually any description or geography of the region until the next century : Strabo, Diodoros but as well Pliny and Caesar. Unfortunately, we don't know how much he mentioned Germans, his works being lost and only known trough extensive quotes and borrowings found in the aforementioned authors, who does give their own descriptions of Germans.

Beyond the Rhine, immediately after Celts or Gauls, going eastwards, live the Germans. There are some small differences between Germans and Celts, in that they are wilder, taller, blonder, but safe all of this they are particularly similar and share a same build, habits and way of life that I previously mentioned for Celts. I think this is the reason why Romans gave them their name : recognizing in them the kinsmen of Gauls and calling them Germani from a word in their language which name the brothers born of a same father and a same mother.

Geography; Strabo

[Marsi, Gambriviani, Suebi, Vandali] are their ancient and true names : Germania is new and a recent additions. The first that crossed the Rhine and expelled the Gauls, and that are now called Tongri, thus called themselves Germani. This name, originally limite to one people, was gradually extended and, created by victory to inspire fear, was soon adopted by the whole nation.

Germania; Tacitus

What we learn there, in spite of obvious discrepancies, is that “German” as an super-group isn’t an indigenous concept, although it was eventually adopted in their relations to Romans, and that it appeared in relations to Gauls.

Interestingly, this is more or less the view of Caesar, as he define Germans in their relations to the “less civilized” of Gauls, such as Belgians (whom he wholly separate from Gauls, in part because they were reputed coming from Germania) and Helvetians, as they fought against Germans that pressed them out of their original territories in both Gaul (where he calls them Germani Cisrheni) or Germania (in the case of Helvetians).

Caesar further describes that Ariovist’s coalition of Germanic peoples were called as mercenaries and auxiliaries against Aedui, illustrating that relations and alliances across the Rhine weren’t unknown (even if it backfired in this precise case), something that would be further pointed out by the Germanic mercenariate (or alliance) with Belgians against Romans or even during early Roman Gaul’s revolts.

This is not the only concession Caesar makes on his radical differenciation between Gauls an Germans, and he have to acknowledge Ubii’s prosperity and habits are relatively close to Gauls, something he attributes to their proximity and trade influence; and as he precises that Ariovist perfectly knew the Gaulish language, we can’t help but notice his own name can be translated in Gaulish as Ariouistos (“Who sees ahead/afar”, roughly).

So far, looking at ancient texts, and even in Caesar’s own accounts, the whole affair seems indeed more complex than Rhine dividing two peoples distinct by their names, way of life, or even language...We need to look at other evidences, firstmost archeological ones.

Southern Germania was the point of origin of the great two material culture (an ensemble of archeological artifacts sharing same features, such as art or techniques) which were dominant in Iron Age Gaul : Halstatt and La Tène. While material and immaterial culture doesn’t necessarily mix (in spite of material culture differences, southern Britons and Gauls seems to have shared a similar language and social features) it does illustrate a similar way of life spread without important difference across the Rhine : the “princely” graves of Vix and Hochdorf belongs to the same civilization, similarly marked by an influx of Etruscan and Greek products, trade and societal influence since the VIIth century BCE. Their regions being connected trough a nodal network of local elites as well as further migrations from Central Europe (mostly to the South and the East, altough probably in northern Gaul as well, cementing a distinct Belgian regionalism) they underwent similar changes in the Second Iron Age and LaTenian periods : the oppida of Bibracte, Manching and Stradonice were connected trough trade roads but also on the same cultural and societal basis, their populations sharing similar rituals (including a shift from inhumation to cremation; or large deposits of buried or drowned objects). Archeologically, southern Germania and Gaul are part of a same ensemble.

This is largely corroborated by the handful of names we have about local rulers and peoples in the Ist century BCE/Ist century AD. Usipetii and Tencterii names are explainable trough Gaulish as “Good riders” and “Faithfuls”, and just as Ariovist, the Marcomanic and Sicambric kings bore what appears to be Gaulish names (Maraubodos as “great raven”, Deudorix maybe as “people’s king”), to say nothing of Germani Cisrheni.

Would it be right, then, to consider their differentiation as artificial and the definition of the Rhine as a border as wholly made-up than XIXth century African borders? In large parts, it does, but it doesn’t tell us all the story.