So would it be a stretch to say that most of the Massalian population are just Gauls who think that they are actually Greeks?
The population in Massalia and the sea line cities is almost completely of Greek origin. In the ex celto-ligurians towns and in the ex Vocontii areas Greeks are the majority. In ex Volcae lands and Aquitani the greeks are small minority exept Tolosa where are the majority. What you propose is the most likely thing to happen if Massaliot League continue to prosper and expand.
Ps: dont forget the 295-293 BC Greek settlers wave from Magna Graecia.