Who is Gabe (the reality) from Next to Normal and what is their importance?

The actual Gabe died sixteen years before the musical begins, when he was approximately two years old. In Act Two, Diana explains to Natalie that he died of an intestinal blockage - when he became ill, he was rushed to the hospital, but died before anything could be done. His death traumatized everyone in his family, particularly Diana, who was pregnant at the time, and Dan, and even Natalie who, born after he died, has had to live with the silent, intense, crippling grief of both her parents.