The motif of PTSD
Many characters in this story suffer from PTSD. Janice escapes because of her PTSD, as does Larry. Amy and Gilbert would love to move somewhere and work through their trauma, but they're trapped in the responsibilities that Janice and Larry escaped. The motif of PTSD is a reminder that the father's suicide was unbearably horrifying to everyone who loved him. It isn't the kind of thing that heals completely in six months or something; it's trauma, and the characters have PTSD because of that.
The motif of hopeless depression
The title says something is eating Gilbert Grape, but which members of the Grape family are not depressed? Amy is certainly depressed. Gilbert, sure. Arnie is depressed and often has fits of agony and emotional torture. Ellen is angry and traumatized, and she is also depressed. The mother is gluttonous as a defense mechanism to defend herself against her emotions, because she is hopelessly depressed about the death of her husband. So the point is simply this: the father committed suicide to escape his depression, but instead, he gave that depression to everyone in his family by traumatizing them. The motif signifies that the whole family is traumatized by his suicide.
Ellen as a symbol for the tragic loss of innocence
Ellen is a symbolic character, because she captures something in her personality that everyone feels but no one says. Ellen is the one who prevents them from pretending that everything is okay, which means that she is constantly in combat with her family who wants to forget the tragic suicide of the father. This leaves Ellen in a position to rebel, and she loses her innocence voluntarily by becoming sexually active. She represents bitterness and neglect, since her mother was never the mother to Ellen that she was to Amy.
Arnie as a depiction of the id
Arnie is a picture of Gilbert's damaged psychology, as well as being his own complete character. As a picture of the id, Arnie represents the human tendency to act out poorly to get attention. He often climbs towers to gather a crowd, which is very id-like behavior. When he wants something, he takes it, like his birthday cake. He represents some of the most fundamental desires of human psychology: the desires for intimacy, pleasure, and attention.
The affair with Mrs. Carver
The situation between Gilbert and Mrs. Carver is dubious at best. Originally, she used his grocery deliveries as a way to have an affair with a younger guy, but Gilbert isn't really satisfied with their arrangement when the thrill of sex wears off. This makes the whole thing symbolic, because it represents that Gilbert is ready to make changes. She represents the life of comfort that keeps him trapped in Endora. Mrs. Carver seems to be an allusion to the Odyssey's character of Circe where Odysseus is trapped in a relationship with a powerful woman whom he likes and enjoys sex with, but ultimately, Odysseus must follow his fate, and so just like Circe, Mrs. Carver gets left behind by Gilbert.
