We don't really know when exactly Gaulish language died out, giving the lack of strong evidence for both a semi-institutional use of Gaulish in Roman Gaul past the IInd century, two main examples being the Stone of Martalis^(1), and the Calendar of Coligny (a Romanized Gaulish religious calendar); and instrumental, everyday, Gaulish with pottery or wood small writing ^(2) sometimes mixed with Latin.
After that, magic writings are the last testimonies of Gaulish, comparable to Bath Curse Tablets, but while in recognizable enough Gaulish up to the IInd century, after that, they're increasingly hard to read either because they're simply badly written or preserved, but as well because more of them are written in a Gaulish-Latin pigdin, if not being mostly Vulgar Latin by the IVth and Vth centuries, with some Gaulish words.Marcellus Empiricus, a medical writer of Burdigala mentioned some Gaulish charms in the midst of his collection^(3).
There's an indication from St. Jerome that Gaulish might have survived at least until the mid IVth in Anatolia and in Rhenish Gaul; if it's not an use of an earlier account, in his commentary of the Epistle to Galatians.
Apart from the Greek language, which is spoken throughout the entire East, the Galatians have their own language, almost the same as the Treveri
The handful of Gaulish words in French, all of them passed trough Vulgar Latin, might be an indication that by the Vth century, Gaulish might have survived there and there as a rustic language, quite degraded and influenced by Vulgar Latin, already dying out, playing to linguistic role in the period.It is possible that Vannetais dialect of Breton might have been influenced by a Gaulish substrate, the difference between a "continental" Gaulish and an "insular" British not being this obvious.
Celtic speeches generally considered, cautiously given the scarce evidence, to be part of the Gaulish linguistic ensemble (such as what was spoken by Noric Celts) are so poorly attested that there's no real way to tell how long they were still spoken in Late Antiquity.
The very last possible mentions of a Gaulish language, identified as gallica lingua is made by Gregory of Tours and Vernantius Fortunatus.
When he [Chroc] came to Auvergne, he set fire to the sanctuary which they call Vasso Galatae in the lingua gallica
The latter gives us an actually good enough etymology of nementon, sacred enclosure
By the name Vernemetis ancient times wished to call [that place] which [in] the gallica lingua translates “mighty temple”
It might be interpreted as a Gaulish survival, but without clear evidence or attestation by these authors, it might simply be the testimony of their erudition.
At earliest, Gaulish might have disappeared by the IVth century, at latest in the VIth, or between both centuries, although probably more early that late.
- Dictionnaire de la Langue Gauloise; une approche linguistique du vieux-celtique continental; Xavier Delmarre; 2003
- La Langue Gauloise; Pierre-Yves Lambert; 1997
- lingua gallica, lingua celtica:Gaulish, Gallo-Latin, or Gallo-Romance?; Alderik H. Blom; 2009
^(1) MARTALIS·DANNOTALI·IEVRV·VCVET·E·SOSIN CELICON ETIC GOBEDBI·DVGIIONTIO VCVETIN IN [.?.] ALISIA translated as Martialis son of Dannotalos gave to Ucuetis this building, with the smiths who honour Uceutis in Alesia
^(2) Such as lubi rutenica obobia | tiedi ulano celcnu; translated as love rutenic thirst-quenchers! or love Rutenicus' thirst-quenchers! followed by a tentative you will be a prince for the feast!
^(3) XI EXUCRI CONE XV CRIGLION AISVS SCRISVMIO VELOR EXVCRI CONE XV GRILAV, which is preceded by a "Gaulish charm" which is rather Latin. (E)Xu might be "go away", cricon/criglion/grilav something stuck in the pipe from Welsh cryg and Cornic greg for hoarse