Priests or priestesses? If the root can mean either, how is gender determined?
It could be either. Or both. If you absolutely needed to specify the sex of the priests, there are ways to do that, but I'm not sure whatis idiomatic. You'd have to ask dedalvs.
In one of the first episodes of the seventh season of the show, they explain that at least some High Valyrian professional nouns are gender-neutral, like the words "doctor" or "lawyer" in English. That's likely the case here.