The full text of Lucian De Dea Syria is online here. cc. 50-51 discuss the galli.
50. On certain days a multitude flocks into the temple, and the Galli in great numbers, sacred as they are, perform the ceremonies of the men and gash their arms and turn their backs to be lashed. Many bystanders play on the pipes the while many beat drums; others sing divine and sacred songs. All this performance takes place outside the temple, and those engaged in the ceremony enter not into the temple.
51. During these days they are made Galli. As the Galli sing and celebrate their orgies, frenzy falls on many of them and many who had come as mere spectators afterwards are found to have committed the great act. I will narrate what they do. Any young man who has resolved on this action, strips off his clothes, and with a loud shout bursts into the midst of the crowd, and picks up a sword from a number of swords which I suppose have been kept ready for many years for this purpose. He takes it and castrates himself and then runs wild through the city, bearing in his hands what he has cut off. He casts it into any house at will, and from this house he receives women’s raiment and ornaments. Thus they act during their ceremonies of castration.
52. The Galli, when dead, are not buried like other men, but when a Gallus dies his companions carry him out into the suburbs, and laying him out on the bier on which they had carried him they cover him with stones, and after this return home. They wait then for seven days, after which they enter the temple. Should they enter before this they would be guilty of blasphemy.
There is a question whether this rite relates to Cybele and Attis, depending on whether we identify the Syrian goddess with Cybele.