What is there to say about Gaz Regans' No. 6 that the man hasn't already said himself? For the whole history of the product, simply visit his website. Here's the short version:
A younger Gaz Regan bemoaned the available options for orange bitters, and took it upon himself to make something better. After five adaptations of a 1939 recipe by Charles H. Baker, Regan and his partners at the Sazerac Company finally had an orange bitters they liked. The sixth attempt was necessary to get it through the TTB (Tax & Trade Bureau), as bitters are supposed to be "non-potable," and No. 5 was a little too tasty for the g-men. Bit of a shame, there.
But the lack of sweetness in Regans' sixth formula is exactly what's made it so famous. Many bartenders bemoaned products that are too candy-like, redolent with that manufactured flavor that smells more like Life Savers than actual orange oil. Regans is instead heavy with cardamom and coriander, and also a blend of the usual suspects including gentian and cinchona.
By the photo on the bottle, it's easy to get the mistaken impression that Regan is some long-dead Union Army field surgeon. But he's in fact alive and well in the Hudson Valley — that is, when he isn't on the spirits competition circuit or guest starring at the Dead Rabbit in downtown New York City. Regan's perceived mythic qualities are simply the product of a well-documented phenomenon called bartender charisma.