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.