- Clerics should be drawn from atypical religious practice
- For religious reasons they are wandering around
- Their abilities should be mechanically linked to their adherence to religious behavior
- Their cult may be popular but being a traveling mystic is always somewhat strange
[NB: I am taking a bunch of creative liberties to make this somewhat workable]
Who are they: castrated individuals who dress in luxurious robes, attractive headgear (crowns, laurel wreathes, or Phrygian caps) and excessive makeup. Their hair is long and their features feminine. They have knives, bronze shields, and tympanum and dance ecstatically. They are preceded by frenzied music and wafts of heady incense. They bear a representation of their goddess atop a donkey. They perform for money. Because of the edict against castration of a citizen, they are drawn from the ranks of low born and foreigners. Sometimes they practice sacred prostitution. The object of equal measures of scorn and religious awe.
Why are they: they wander around and perform blessings (a shield dance over newborns, blessing crops to be fertile as Attis represents the life-death-rebirth cycle of crops) and tell fortunes. The actual operations of the cult are opaque (it is, after all, a mystery cult).
Domains: the boundary between life and death, the wild (particularly birds of prey and lions), frenzied dancing, mountains and stones in general. In chthonic ritual practice can contact an ancient race of smiths/magicians who can work metal, teach the secrets of mathematics and literacy and bestow magic; they will also witness oaths and bind them.
Ways of winning divine favor: Convert followers to worship, observe the holy week of Attis being pledged to Cybele, being castrated, dying and being reborn, make a sacrifice of a ram to Attis (Criobolium) or a bull to Cybele (Taurbolium), ritually sacrifice the testicles of a potent animal, observe minor holidays organized around the import of Cybele's worship. Flagellation and frenzy demonstrating devotion. Give solid agricultural advice in line with the religious calendar.
What are their powers: Lions will not attack them (so long as they are playing divine music) and can potentially be commanded (birds of prey are the same), they can make things fertile, they can communicate with powers under the earth to learn the art of metallurgy, magic, mathematics or literacy, they can shape stone and commune with the dead. They can perform a shield dance and enter a battle frenzy.
How is one initiated: the only requirement is the devotion necessary to castrate oneself in a fit of divine madness during the 'Day of Blood' (March 24) in symbolic mourning for the temporary death of Attis. Then you become a mendicant priest and perform for the glory of Attis & Cybele.
 Galli are in various sources referred to with masculine pronouns, feminine pronouns and 3rd gender pronouns sometimes pegged to if they have castrated themselves.