memes

I tried to fact check the original anecdote. Turns out there is a source for this, it appears to be from Historia Augusta, a collection of biographies of Roman emperors in the 2nd and 3rd century CE. However, it's highly debated about how reliable the work is — when it was written is highly debated but it seems to be from the 4th or 5th century CE (which by itself isn't too big of a problem), and the author seems to be using it to make points about contemporary issues, sort of as a parody. It's not clear to modern scholars which parts are grounded in the sources and what has been added for flair .

Anyway, here's another story that immediately precedes the chicken one in Historia Augusta:

“Gallienus was, moreover, exceedingly clever, and I would do well to add a few examples of his sharp wit here. Once, when he had a huge bull sent into the amphitheater and the gladiator who was sent to kill him proved unable to kill the bull even after being brought out ten times, Gallienus sent the man a crown. When everyone began to murmur and wonder what the world was coming to for a totally incompetent person to be crowned, Gallienus ordered a herald to announce, ‘It is a difficult thing not to wound a bull so many times.' "

Based on that, actually, I wouldn't be surprised if the merchant in the original story was allowed to live. Partly because as you say, the humiliation may have been seen as punishment enough, but also it's plausible that the merchant ended up being a sort of monument to Gallienus' cleverness. This is just speculation though — if the author of Historia Augusta took liberties with history to tell a good story, then I see nothing wrong with my fun headcanons