Gravity's Rainbow Title
In Genesis 9:13-15, the first rainbow is a token of God's covenant not to destroy humanity again -- or not with a flood, anyway. As an old spiritual has it: "God gave Noah the rainbow sign / No more water, the fire next time."
One reading of the title is that "gravity's rainbow" is the parabola of a ballistic missile's flight, halted on the final page a "last unmeasurable gap... the last delta-t" over our heads: fiery destruction held in abeyance. For now.
The connection between the parabolic flight path of the Rocket and a rainbow is made explicit in an important discussion of the rocket's trajectory (p. 209):
"But it is a curve each of them feels, unmistakably. It is the parabola. They must have guessed, once or twice - guessed and refused to believe - that everything, always, collectively, had been moving toward that purified shape latent in the sky, that shape of no surprise, no second chances, no return. Yet they do move forever under it, reserved for its own black-and-white bad news certainly as if it were the Rainbow, and they its children...."