I have recently learned about the Gallic Cubi-Biturige, whom were based on the site of modern French Bourges. Apparently, for a brief period in 600's BCE, they ruled over a tribal confederacy so powerful, that they practically unified Gaul for a time.
I'm tring to find some proper authoritive sources Online, but embarrassingly, the only forum with the most extensive information is the Europa Barbarorum website. Other Celtic history forums only seem to hint vaguely at it, so I'm not sure if this was a historical reality or a modern myth.