Gretta is an expedient who has the function to activate the epiphany. In this way she allows Gabriel Conroy to gain a deeper level of awareness. Before epiphany Mr Gabriel is emotionally dependent on his wife, indeed he is paralyzed: he doesn't reach the belance beween his reason and his emotions. When Gretta tells him the story about Michael Fury he has a bad opinion about him but then he realizes he isn't able to give her wife all the love Michael Fury gave her.
The name ephipany comes from the religious festival called Ephipany that celebrates Jesus Christ's revelation to the magi. Jesus Christ represents the transformation of the sin in salvation.