Gaulish being around in the 9th or 10th centuries is a bit fanciful. As far as I know direct sightings end during the 5th century and circumstantial evidence ends during the 6th c., at the same time Christianity became truly ubiquitous even in remote areas and Latin was solidified as the only cultural language. It makes little sense for Gaulish to have survived much longer.
There's certainly a case to be made for some dialects of Breton having a Gaulish substrate (after all Gallo-Romance languages all have it and that mutation occurred around the time Bretons migrated into Gaul).
Yeah the paper I linked to above is pretty critical about the idea of late attestations of the language generally. Even seems to suggest nothing truly concrete beyond the 3rd century. So leaves it fairly ambivalent about whether there is truly Gaulish influence into Breton.
There's certainly a case to be made for some dialects of Breton having a Gaulish substrate (after all Gallo-Romance languages all have it and that mutation occurred around the time Bretons migrated into Gaul).