I am confused about the musical "Next to Normal"?

Hey! This bothered me for a little while, too. I read and reread synopsis' but I just couldn't figure it out. Eventually, I just listened to the cast recording over and over again, and found it. THEN I read synopsis' more, and found it. Gabe is a spirit - Diana's hallucination. I am almost positive that everything he does is either a hallucination of Diana's, or he's talking to the audience ("Aftershocks", for example). As for his interaction with his dad, Dan, I think that since that's technically after Di leaves, he might be just calling out to his son, as in saying, you know "Diana used to talk to you, and it helped her. Can you help me?" type thing. I don't think that communication is successful, but it's an attempt. I hope this helps! I am a HUGE Next to Normal fan.