What is the difference between a cyclocross bike and a gravel bike?

The key differences are geometry and intent. CX bikes have higher bottom brackets (65–70mm drop vs. 70–75mm for gravel), shorter chainstays for agility, and geometry tuned for 40–60 minute race efforts on tight courses. Gravel bikes have lower BBs for stability, longer wheelbases for comfort, more tire clearance (45–50mm+ vs. 33–40mm), and more mounting points for long-distance riding. CX bikes are race tools; gravel bikes are endurance machines. For casual mixed-surface riding, a gravel bike is usually more comfortable; for racing or aggressive riding, a CX bike is more responsive.