Within the cult of Cybele (or Magna Mater as it's more appropriate to call her if we're talking about her cult center within the city) the Galli were the only self-castrates. The Galli were not priests, but rather wandering beggars--the Phrygian version of the cult of Cybele in particular had a number of different kinds of these, including the Galli and the Μητραγύρται. Castration among the Galli was probably much rarer than Catullus 63 would suggest--it doesn't even appear until the 2nd Century, B.C., after the cult of Magna Mater (which in many ways is very different from the Phrygian cults) was already established in the city. Galli were certainly not all eunuchs, and it seems that the vast majority of self-castrates were already Galli, further initiating themselves into their service to the goddess--the actual priests of Cybele were also castrates, but not in the ecstatic rituals of the Galli, in which they did not directly participate. The Galli and other itinerant beggars in the service of Cybele (who referred to themselves as her slaves, δοῦλοι) wandered around in poverty begging alms and dancing ecstatically during her public festivals. In the city the cult of the Magna Mater was extremely isolated, however. Roman citizens were forbidden from becoming priests of Cybele and her priests were always imported, so to speak, from Phrygia--even Italians don't appear to have been priests of Cybele. Until Claudius established the archigallus, the chief of the Galli who was always an (uncastrated) Roman citizen, Roman citizens could also not be Galli or castrate themselves (self-castration was illegal no matter what to citizens). Magna Mater only had a handful of festivals, the most important of which was the ludi Megalenses in early April, which was mostly celebrated by the aediles, the urban praetor, and some of the patricians, rather than by the priests of Cybele and the Galli, and which consisted pretty much only of a procession and a ritual bathing of the cult image. There was an earlier series of feast days, for about a week in mid-to-late March, which focused more on Attis than Cybele--during these the Galli were more involved, especially in the ecstatic rituals on March 24. Other than March 24 though it was rare to see the Galli doing their frenzy thing. Galli were only allowed to walk the streets to beg for alms on particular days, and the cult of Magna Mater didn't really get out of her temple on the Palatine much anyway. Galli were not the only initiates into the cult of Cybele--indeed, one might get away with not even calling them initiates at all, since they were never initiated into her (or rather Attis') mysteries. Actual initiation into the mysteries involved a ritual that we don't understand very well--something involving the taurobolium, a bull sacrifice whose descriptions are heavily colored by later Christian polemic. But the Galli were not like that--their self-identification as slaves of the goddess speaks to how they fit into the cult.