just wiki'd
Galli (singular Gallus) was the Roman name for castrated followers of the Phrygian goddess Cybele, which were regarded as a third gender by contemporary Roman scholars, and are in some ways like transgendered people in the modern world. The chief of these priests was referred to as a battakes, and later as the archigallus.[1]
Cybele's Galli were similar in form to other colleges of priests in Asia Minor that ancient authors described as "eunuchs", such as the priests of Atargatis described by Apuleius and Lucian, or the galloi of the temple of Artemis at Ephesus.
The first Galli arrived in Rome when the Senate officially adopted Cybele as a state goddess in 203 BC. Until the first century AD, Roman citizens were prohibited from becoming Galli. Under Claudius, however, this ban was lifted.
Further information is difficult to come by, given the persecution faced by followers of Cybele and other pagan deities after the Theodosian edict of 391 AD. All of her temples were destroyed, with orders that they should never be built upon (in contrast to the usual practice of converting non-Christian religious sites). As a result the only surviving records of the Galli come from historians and archivists. The accuracy of such records is often dubious because of the gender biases of most ancient writers.
The name Galli is probably derived from the Gallus river in Phrygia. One of the first temples to Cybele was built near this river, which led to a rumor that drinking from the Gallus would cause such madness that the drinker would castrate himself. It has also been supposed that Galli is derived from the Latin word for rooster. Hieronymus believed the name was given by the Romans as a sign of their contempt for the Gauls. However, in that case, gallus would have been borrowed from Asia or Greece, where it meant eunuch.
The Galli were castrated voluntarily, typically during an ecstatic celebration called Dies Sanguinis, or Day of Blood, which took place on March 24.
