I've read that the Gauls were known at the time to cultivate hemp and some Gaulish communities became quite wealthy trading it with Mediterranean civilizations, but is it known if they ever discovered the recreational use of it?
Cultivation and use of hemp in pre-Roman Gaul is an obscure topic, due to the limited archaeological sources. In fact, until the 2000's, these were utterly absent (although traces were found in southern Germania, participating to the same material culture as Gaul) and its introduction in the region was generally attributed to Romans but evidences or hints of its presence and cultivation were found since.
Namely achenes (deposits from harvest, preparation or uses) in the valley of Al Poux (Lot) as well as at Saverne and Schaeffersheim (Bas-Rhin), ropes at Lattes (Hérault), Eberdingen-Hochdorf (Baden-Wrutemberg), Halein (Austria) and traces of deposits in a wine recipient in a grave at Cébazat (Puy-De-Dôme); along evidence for wild hemp and its use in temperate Europe since the Mesolithic.
It's not much, especially as cannabis achenes can easily be confused with humulus or hop with the distinction being largely contextual (i.e. the latter being favoured in a foresty context) : the lack of large deposits, ancient literary mentions or specialized tooling rather give the idea of a small scale household production compared to the medieval era. As far as I know, no pre-Roman people was renowned for it in ancient sources, nor archaeological sources evidence that it might have been the case.
But along the introduction of Mediterranean plants such as grape or olives in the region since at least the Vth century BCE, it wouldn't be too much of a stretch to wonder whether hemp wasn't introduced there by ancient Mediterranean traders (Phoenicians, Etruscans, Greeks, Italians, etc.) and locally cultivated and/or imported.
What hemp was used for, however, is not something easily addressed.
Textile is an obvious possibility, especially for rope and sails, giving it was one of its main proto-historical and historical uses as accounted for by ancient authors (e.g. Pliny; Natural History; XIX-56)
Alimentary uses aren't found in pre-Roman Gaul but can't be excluded as evidenced in ancient Greek and Roman sources while not for Gaul itself especially in the light of their high-oil component. It's possible, still, that the achenes founds on the site of Al Poux might have been used as baits for fishes.
When it comes to both alimentary, ostentatious and recreative use of cannabis sativa, its traces recently found in 2015 in a wine jar (and yes, pretty much all jokes about the magic potion were done) deposited in a notable's grave nearby a sanctuary are very interesting although the question remains open whether the cannabis was infused for its psychoactive values or not.
It was common to mix wine or beer with some ingredients to both better preserve the beverage (especially wine, with virtually anything ranging from seawater or resin to plaster) and enhance its flavour (with various plants, herbs and fruits being used as gruit in beer before hop became commonplace).
But while it could be merely a matter of taste or experiment, it's not impossible a psychoactive was known or searched for by Gauls.
It was after all a known property of cannabis by ancient authors as Herodotos, Dioscorides, Pilny, Gallian, etc. along other psychotropic plants (opium, belladona, etc.) and it's not impossible although the mentions of cannabis in Greek-Roman pharmacopeia are fairly tardive (by the turn of the millennium) it's far from unthinkable that Gallic practices did not incorporate hemp trough the Greek-Roman contacts during the Late Iron Age, druidic pharmacopeia being partly accounted for in Pliny's Natural History.
It had been argued that the addition of psychotropic ingredient was not uncommon in the prehistoric or ancient world (poppy seeds nightshade, hemlock, belladonna, etc.) and that cannabis wouldn't have been an exception, especially with such practices being associated with both healing and divinity (which, in the context of Orphic mysteries and their use of psychoactive fumigation and their possible connection to Druidic practices could provide interesting insight); whereas these ancient physicians and herbalists complained while mentioning these plants that people could abuse or build a resistance to them, hinting the distinction between healing, ritualistic use and recreation might have been fairly blurry.
This is of course merely speculation, with an ongoing set of considerations about the use of cannabis amongst ancient Greek and Romans, that many still consider to have been fairly unpopular outside specific social contexts, but it's worth considering to give some light to this interesting finding.
Would this person, as we don't know their gender or their social position (except they might have been fairly wealthy), have carried cannabis-infused wine in the Otherworld because of its taste, because it was pleasant, because it helped them curing their ailments at the age of sixty years old or because it was a way to enter in a trance similar to their journey beyond? Or maybe all or part of that?
There is, unfortunately, no way to definitely answer no more than whether or not it was common practice : as it is unattested by ancient authors, otherwise avid to write about any kind of "exoticism" they could mention, it could be as simple as a Gaulish member of the elite drinking and being supplied with wine in the Roman fashion. The traditional symbolism of the wine might have associated with blood and thus called for its inalteration, but as Roman influence found its way among late Gaulish elites, private consumption departed from that; and eventually displacing the question from "did Gauls drink a cannabis-infused wine for its recreative purposes" to "did Romans do that".