The title implies the main themes of the book quite well. Once we learn that Gloria is actually a strange psychic manifestation in Jimmy's imagination, the title pretty much fills in the rest. Jimmy is using women for his personal journey through the horrors of loneliness, detachment, and PTSD, but because he treats women as religious fetishes (literally, he worships them as idols of "Gloria") the women end up connecting with Jimmy because he likes to hear their stories.
This novel turns out to be a Jungian analysis of sexual addiction due to PTSD. By analyzing Gloria through the archetypal lens of Jung's "Anima" we can see that Jimmy is having an Oedipal nightmare. Instead of viewing the Oedipal state like Freud, Jung argued that an Oedipal nightmare is what happens when a person feels compelled to regress toward a state of infancy (they want to go back to when they were children, when they had no responsibility or experience of trauma). Jimmy demonstrates these ideas with artistic precision.
This book is obviously informed by those theories. It also seems to demonstrate Jung's idea that the easiest way to help a person through psychic torment is to encourage them to be successful in whatever delusional game their brain is playing. Jimmy does this by truly connecting with each prostitute he hires. Because he is genuinely curious about and attentive to the women, and because he treats them as goddesses of his own fortune, they respond kindly to him.
Jung would say that he has succeeded in the delusion by learning how to become successful with women. And, as Jung predicts, Jimmy realizes that Gloria is part of himself, and he realizes that the truth about reality is far more complicated than it looks.
