The doctor visit
When the family needs to visit the hospital, Gertie has a hard time finding a ride. Finally, she convinces a stranger to help her, a symbol for the lack of community in her neighborhood. She finds a way to the hospital, but by the time the doctor is there to see her, the son has improved. He was truly sick and in need of medical attention, but instead of getting the help he needs, the boy pushed through his illness to feel better. In other words he was embarrassed about needing help, and that is what this symbolizes about Gertie as well.
The Bible
Gertie is a Bible-believing Christian. She isn't just a religious person either. He intricate knowledge of the Bible shows that she has true character. She doesn't just believe because someone told her to—instead she had the tenacity and curiosity to explore the book herself. This becomes a symbol for a religious encounter with fate, because she is good, and she is well-read, and she loves her family with sacrificial love, and yet God does not bless her with a kind, pleasant fate. Instead of being a "religious" person, the novel slowly drains her of joy and hope until she is in absolute crisis. The symbolism is clear; religion is not a way of reducing life's suffering. For Gertie to love God is for her to accept life's torturous fate. no person can easily do such a thing.
The Tipton house
The house on the hill is a symbol for the Promised Land, or a symbol for paradise. She wants a better life for herself and her children, but there isn't a realistic path to make that happen. If she is to find paradise, she realizes that she might not be able to attain it with unrealistic goals. Despite that, she cannot help but to hope beyond hope that one day, they would get a chance to see the good life, the life that is enjoyed by wealthier people with kinder fates, for instance. This house represents her desire.
Detroit and hopelessness
If the Tipton house is her paradise, then Detroit is Gertie's wilderness. Like the Israelites who fled slavery to die in the desert, Gertie is also asked to leave suffering to enter a new, different suffering that is in some ways more chaotic and unpredictable. They cannot make a new life unfold for themselves in Detroit. This is partially due to the way war has changed the local economy, and partially due to the imbalance caused by the draft and the deaths of young men.
The death of the daughter
When the daughter dies, that is a symbol for Gertie's hope dying as well. Just as a child is an incarnate chance for new life, the death of a child is a symbol for the absolute sureness that life will not continue. She knows that without her children to fight for, she has nothing left to strive toward, nothing left to accomplish, and she begins a decline to true hopelessness. She gives up her symbolic sculpture, which is a symbol for the work of her hands. She stops trying.
