The narrator's tone in Go Tell it on the Mountain is pretty subdued. There are no tirades against religion or Gabriel: the narrator does let the characters do the talking. We find out all about Gabriel's missteps through Florence's words and Gabriel's thoughts, so the narrator doesn't really have to judge, but there is some smugness going on there.
In the beginning of the novel the narrator says that "Roy would be like [the sinners] when he grew up, if the Lord did not change his heart" (1.1.4), but it really sounds more like something that Roy's parents would say. The narrator is repeating it in a tongue-in-cheek manner.
In another smug-narrator move, we see that when Elizabeth moves to the city with Roy, the narrator uses her reasoning but in such a way that we all know what's really going on (i.e., Roy is never going to marry her):
Richard said that they would marry as soon as he had saved some money. But since he was going to school at night and made very little money, their marriage, which she had thought of as taking place almost as soon as she arrived, was planned for a future that grew ever more remote. (2.3.72)
The narrator doesn't take on Elizabeth's voice here, but does relay her thought process. The reader gets the idea that she's being taken for a ride, and that she sort of knows it but doesn't want to admit it to herself. The snarkiness, though subtle, shines in moments like these.