Rabbi Gedaliah ibn Yachya in his work Shalshelet ha-Kabbalah (Jerusalem, 1962), p. 223 alleges that when sacrificing animals, some pagans would remove the gallbladder (מרה, marah) from the animal in order to symbolize that they request sweet things from their god, not bitterness (מרה, marah). Is there any evidence of such a practice in the ancient world? If so, which cult(ure)s followed such a practice?
My esteemed brother-in-law just emailed me an answer. He wrote:
Shalsheleth Haqabbalah may not be as good as a bona fide dead clock which (assuming it is analog) is exactly correct twice a day, but note Clement of Alexandria in Protrept. I 2, PG VIII, 76a.
Although there we see the removal of the heart, the consumption of bile may well be associated with the gallbladder. Perhaps it is my ignorance, but I thought that מרה was bile. If so, you may have your source. The question is whether Clement referred to the consumption of bile specifically from sacrifices, (whether the removal of the bile was itself a form of sacrifice, etc.)
For an old English translation of the Greek, see here: https://en.wikisource.org/wiki/Ante-Nicene_Christian_Library/Exhortation_to_the_Heathen#28
So there it is!