Gravity's Rainbow is more relevant today than ever. People construct their own reality and people need a constructible and explainable reality, some kind of answer to the chaotic nature of life.

The main theme from “Gravity’s Rainbow” is the need for people to create a controllable reality and the effect on them if that is not possible. WW2, in the novel, destroys the sense of control that people have. Early in the novel, the V2 rocket is an agent of that loss of control. It plummets at supersonic speed. People feel helpless in the face of it and psychologically fail. And with equal measure the rocket means Power.

The characters in the book seek new beliefs, new constructs, anything to regain that control. Science, mysticism Love (Roger Mexico and Jessica) shown in the novel to be mechanisms whereby people seek that control. Later in the novel, the rocket is shown to have taken on mystic significance and is incorporated as part of the psychological explanation that provide people with the illusion of comprehension and control of their surroundings. The Hereros want the rocket to gain power, as a powerless people.

The war, in the novel, destroys the sense of control that people have, if people actually have power. Really most of us, if not all of us, are cogs in the machine. Gravity's Rainbow addresses this existential horror.

People feel helpless in the face of it and psychologically fail, spiritually fail - want to give up, as Slothrop did (he dropped out, like Pynchon, he disappeared).

We seek new beliefs, new constructs, anything to regain that control. Science, mysticism(Seances), relationships(Mexico and Jessica), and anything are shown in the novel to be ways whereby people seek that control. Later in the novel, the rocket is shown to have taken on mystic significance and is incorporated as part of the psychological explanation that provide people with the illusion of comprehension and control of their surroundings.

I also think that Slothrop did get a happy ending, at least somewhat. He may be "scattered", and never fully able to let go of the ideal of America sitting in the back of his brain, but he's also as free as it gets - he's fully disconnected from "Their" system. The Hanged Man definitely connects to him in this sense, because his Self was sacrificed, but that was the only way he could survive.