Gauls is a better term, "French Celts" is a bit weird since the name France derives from the Germanic Franks and French culture itself is a Romanized one at its core.

That being said, I find it unlikely to have been from Gaul. "Halloween" is a very North American and UK-Ireland type of thing, not something you find in the rest of Europe. The rest of the continent celebrates All Saints' Eve without costumes or apple bobbing and whatnot, instead they go to mass and pay their respects to dead family members.

In France, Spain, Portugal (all areas formerly inhabited by Celts the Romans conquered), Halloween traditions like trick or treating and costumes are seen as Americanisation and foreign.