There is some connection between the king cake and the idea, as presented by Sir James Frazer in The Golden Bough, of choosing a mock king by lot.

So the idea is ancient and probably has its roots in someplace other than Gaul. In fact, I'm not sure it's even possible to say where the idea originated, given how pervasive it is.

But there are common themes running through its appearance, no matter where that might be. The king for a day concept ties in with Mardi Gras, where the Lord of Misrule rules over the raucus celebration for that day only.

And there also is the idea of mocking the prevailing social order, so that the world is temporarily turned upside down while this bogus king holds sway.

I've also heard of a practice among Orthodox Christians of eating something called St. Basil's bread. It has a lucky coin inside, and the person who gets the piece with the coin is supposed to have good luck all year (assuming he didn't swallow the coin. They also eat this bread on the feast of the Epiphany, which just happens to be the day a lot of Old Calendar Orthodox celebrate Christmas.