Gaulish society by the -60 was actually actively anti-royal, at least in Celtic Gaul (which roughly was delimited by the region between Rhine, Marne, Seine an Garonne) : Celtillos was condamned to be burned at the stake after having been found guilty of conspiring to restore Arvern royalty (which was toppled in late -120's, after Roman victory over Bituitos), Orogoterix preferred to kill himself rather that being liksewise executed, Tasgetios was murdered after Caesar imposed him as king in -56, Dumnorix was able to raise suspicion about Romans amongst Aedui claiming Caesar proposed him the royal title, etc.

Neither Gaulish petty-states were limited to tribes (or teutas) : Caesar barely mention these and rather focuses on what he call civitates, which is not litterally cities but in the Late Republican sense rather coherent human groups forming a polity. These cities were themselves formed of pagi (cantons) which were formerly usually considered as tribal territories, but now more looked at as administrative, military and fiscal units benefiting from a strong autonomy, themselves subdivided in "fractions of pagi" (Caesar) which might be tribal entities, with maybe one tribe in particular dominating it.

Civitates, as federation of pagi, were quite decentralized but ultimately were ruled by oligarchic assemblies (senates) made of equites (aristocrats) whom prestige depended as much of their military role than the constitution of a clientele, a personal/familial constitution of a following just as in Rome; and factions can be observed (and Caesar describes them as such) to be a political/personal split in Gaulish society, with a conservative and a populist part less ideological than programmatic in nature (again, not unlike Rome).These civitates formed regional ensemble of two kinds : first a proper diplomatic and politic network where powerful people dominated weaker peoples in a patron/client relationship or tied alliances comparable to political, military or monetary unions from one hand; and resorted to regional (Belgican and likely Armorica, maybe Celtica) and critically pan-Gaulish assemblies (concilium totius Galliae, and the annual Druidic assembly) which looked a bit like an oligarchic high-kingship.

Ariovist' power most probably came from leading a coalition, and his title of king is rather non-descrptive (and might be better translated as "a king of Germans"). especially as "Germani" covers more a geo-social concept at this point, than being entierely Germanic in the ethnic sense. Probably, he was leading a mix of Celtic and Germanic groups, his own name being a Gaulish one. The comparison with Late Ancient Barbarians isn't necessarily ill-fitting but only if we're considering Barbarians before they build up states largely independent from the Empire in the Vth century, as a politically coherent group of people. Even there, Ariovist largely build its coalition and political ascencion using Gaulish political conceptions, searching to obtain a regional primacy (basically being the first people in pan-Gaulish assembly).

If you're interested on details, and even if quoting yourself can be a bad form, there's some additional details Gaulish identities and politics on this subreddit.

Conversely, what Barbarian kingdoms looked like? Well, unsurprisingly, societies and politics separated by 6 centuries of Roman cultural dominance didn't looked the same.

Barbarians, by the IIIrd century, can't really be considered as being primarily divided on tribes, weather on the other side of Hadrian's Wall with Picts; along the Rhine and North Sea with Saxons, Franks, Alamans, Burgundians; along the Danube with Marcomans, Quades, Asdinges, Iazyges, Thervinges, Greuthinges, etc. to name but the famous ones were new, more important groups made up of local tribes and first coalitions, and with newcomers from eastern Germania or from the steppes (mostly Iranians as Sarmatians, but probably a mix of proto-Balto-Slavs and what remained of Dacians too).These coalitions were made to allow frontier's people to benefit from the weakening of the European limes, due to the necessities of the war against Persia's dynasties. They coalesced quickly into politically defined groups, in no small part because of Roman management or negotiation with them, treating these groups as (inferior) interlocutors being either defeated and deported within the empire as civilian or military colons (dedicitii, altough laeti is growingly used by the IVth century, especially as they are not scattered systematically anymore); either bought as allies or mercenaries trough subsides and payment for the service.

Long story short, by the late IVth century, these peoples relatively united under a regulus (petty-king) who ruled over other chiefs and a Barbarian people that defined itself first-most as all who obeyed this petty-king (due to raids and enslavement of romanized populations; and being joined by military deserters, as well as civilians; it included perfectly romanized populations). Some of them began to enter in the Empire as such groups by the mid-IVth century : at first Franks in Northern Gaul and Goths in Moesia. For the first time they formed a new kind of settlement where peoples were trusted a large territory comparable to a province. This was followed by new migrations as Hunnic pressure became a reality with, for exemple,Burgundians which were authorized to settle : with Vandals, Alans and Suebi forcing their way in in the early Vth century.

These peoples, increasingly romanizing, took more and more strategic importance as they were particularily militarized and being largely used as semi-professional armies. "Barbarisation of the Roman Army" is somewhat of a trope, and without being a wrong way to look at it, it must be remembered that auxiliaries and irregulars often represented a large part of classical Roman armies : the difference there is that Barbarians more and more doubled as an autonomous political ensemble (although Barbarian serving the empire as laetic groups or even as Roman citizens such as Merobaud or Stilicho was a common sight since the IIIrd century). It led to inner strife with Romans (who ended up using other Barbarians against Barbarians, for instance Huns against Goths, then Goths against Huns, then in the East, Huns against other threats), with a cycle of negociations, settlement, war, negociations, settlement, etc.

The early Vth century saw two phenomenons : the limes breaking up in the West, and the settlement of Barbarian peoples as autonomous states which took over the local militia (administrative and military public service) and provincial management and underwent a territorial build-up on a less and less powerful Roman authority which under Italian Roman elites and Ricimer went trough a "Italy First!" policy, while it disappeared in Britain,Illyricum and Northern Gaul. Basically the empire falling into sheer warlordism, Barbarian (Euric, Gaiseric, Rechila) or Roman (Aetius, Victorius, Marcellinus).

By the late Vth century, Barbarians essentially took over western provinces as Roman states, with the participation of local Roman elites, as the imperial idea mostly was that at this point : an idea; and felt more loyalty since decades to the emperor or the palatial court, than provincial or even Roman state apparatus itself.
There were of course regional differences (depending where Roman political structures were more resilient : obviously more in Italy and Africa, less so in southern Gaul and Spain, and much less so in Northern Gaul, and virtually disappearing in Britain or Illyricum) and subsequent evolutions from the Roman model in a situation of economic decline), but Barbarian states are better understood as post-imperial Roman states.

- Barbarian Migrations and the Roman West, 376-568 Guy Halsall
- Late Roman Warlords; Penny MacGeorge
- Les peuples Gaulois (IIIè-Iè siècles avant J.-C.); Stefan Fichtl
- Servir l'Etat Barbare dans la Gaule Franque, Bruno Dumézil