[Trigger Warning : including, but not limited to, detailed post-mortem decomposition and mutilation, human trophies, cannibalism, suicide, etc.]

Sacrificial practices in Gaul between the IIIrd and Ist centuries BCE are known through two sorts of sources, archeological and historical, which give us a complex, contrasted and sometimes contradicting view of Gaulish religious rites.

It's worth pointing out that the bulk of sacrifices that are attested archeologically are largely non-descript in classical sources maybe due to their lack of exoticism as they were largely similar with the sacrificial rites in the other parts of the classical Mediterranean world : as such, animal remains found in Late Iron Age Gaulish sanctuaries are essentially fitting the hierarchical list of sacrificial animals in ancient Indo-European peoples' religions (horse, cattle, sheep, goats or, as in Roman sacrifices, pigs), including a relatively common sacrifice of dogs, albeit in various proportions depending of the sanctuaries. Wild animals are remarkably absent of such deposits being associated with the divine sphere to begin with, and whose aristocratic hunt had to be compensated (depending on the worth of the animal, associated as an animal equivalent to the warrior-aristocrat) with an annual animal sacrifice.

Most animal reminders in sanctuaries (from seemingly young animals) aren't really distinct in the state they're found from the food reliefs found around habitations and are actually the vast majority of cases, hinting at communal feasting by the locals and sometimes their dogs. This again would make Gaulish rites close to what existed in ancient Greece and Rome where a division of parts consumed by men and parts owed to deities (probably fatty parts as well, long bones, offals and maybe brains at least in the sanctuary of Gournay s/Aronde) was performed, remainders of the former being deposited or pushed back to the enclosure of the consecrated space. Apart from a brief mention in Pliny (Natural History XVI, 95) of the sacrifice of white bulls in relation to mistletoe harvest1 the lack of contextual information makes what was probably a broadly common form of sacrifice (although with strong regional variations) largely unknown to us, unfortunately : but by comparison with classical rites, we can hypothesize they had a propitiatory or thanksgiving nature, contrasting with chthonic offerings.

These involved votive offering and sacrifices to a subterranean deity, probably identifiable as the Gaulish progenitor Dis Pater mentioned by Caesar (DBG, VI, 18), or to divine aspects related to such as funeral or mortuary rites and heroic celebrations (notably linked to the possible notion of reincarnation through a subterranean space of creation/re-creation). Animals (generally aged oxen, cows or bulls; likely horses) were brought before concave altars and slaughtered or stroke on the skull with an axe or hammer (and, in at least one case, with a spear), their blood serving as entire libation in some case whereas the whole body could be thrust within the wheel and left to rot possibly as the food poured to the underworld until the carcass was removed, its bones cured and sorted out (most of it being put in the ditched enclosure skulls being probably set on display at the porticos for years before being ditched out, at the exception of long bones whose destination at this point is unknown).

Besides animal sacrifices, there were obviously many other sorts of sacred offerings, besides funeral deposits (which aren't necessarily much different) whose historical attestation is more or less limited but benefits from increasing archeological discoveries : libation of wine poured in concave altars, buried deposits of everyday artifacts (axes, jewelry, etc.), display of armors and weapons inside sanctuaries , votive deposits of gold or precious objects (in lakes, associated with the underworld, or sanctuaries), cultual cakes, grain and other vegetal offerings (and possibly, as the golden tree of Manching, man-made offering mick-miking nature), etc.

Comparatively to both the archeological evidence for animal sacrifices, and the lot of literary classical accounts, traces of human sacrifice in Gaul are far from obvious, especially as human remains can be found in a different social and religious context, albeit in fairly goulish fashion from a classical perspective.

By far the most popularly known practice was head-hunting, fairly well known as a Gaulish feature by Greek and Roman authors (e.g. (Diodorus Siculus, V, 29, 4-5 or Strabo, Geography; IV, 4, 5) but also well evidenced by archeology with skulls and skulls remains with traces of decapitation and outdoor display, along limited indigenous artistic evidence or head-shaped holes in posts or porticos. Their sight seems to have been fairly ubiquitous in Gaul, being both the mark of a social status for a warrior-aristocrat (as proof of military prowess but as well as a proto-monetary token along with torc appropriation) but also as being a recipient of vital energy (*anation?) appropriated from a defeated enemy to the benefit of the head's possessor or the community : indeed, besides domestic preservation, there's evidence for their display in public spaces associated with public civic and/or religious rites as sanctuaries' porticos (but as well fortified walls or public squares).

The piles of broken human bones, associated with broken or twisted weapons, found in some enclosures can be explained through a related display. Both Greeks and Romans established “trophies” (tropaia or tropaea) to celebrate important victories over their enemies, and Greek especially set them on the site of the battle, ideally on the place the defeated army broke down ranks and turned back (tropê, hence the name). But while Greeks were content with piling up weaponry, Gauls took the whole bodies, after having consciously beheaded them in a process that could take days, maybe leading in some cases to smoke the corpses, and carried them to the consecrated space. Yet contrasting to the Greek practice, the Gaulish trophy was built along the specifications of other sanctuaries (with the difference that they were positioned westwards rather than eastwards) as a perennial site of celebration. Immediately outside the enclosure were built covered platforms on which were set the bodies were suspended, left to rot and dry, whichever part happened to fall remained untouched, for an indeterminate period, forming an atropopaic guard. After a while (maybe one year), reminders were carried inside the enclosure and separated (being putrefied enough that separation did not seem to have involved cutting them) and whatever flesh was still there removed, the bones being broken, fired, and poured in an concave ossuary whose edge was made of human and equine bones until they were filled.

Not all presence of human bones in trophic spaces were necessarily enemies vowed to the divinity having presided to the victory of the in-group, especially in a context of sky-burials where the flesh of warriors was carried by vultures, crows and other birds have support in ancient literature (Sillus Italicus, Punica, III, 340-343) indigenous representation but as well archeology, associated with an heroicising cult over the whole period in situ (while all traces of the structures in which it happened disappeared).

Not to discourage any further answers but /u/Libertat gave a really illuminating answer to the burning question of whether or not wickermen were ever actually used in Celtic Gaul & Britain.