Geometry is actually probably the big difference - cyclocross bikes are designed for small repeatable courses often with tight turns and the need to carry the b

Geometry is actually probably the big difference - cyclocross bikes are designed for small repeatable courses often with tight turns and the need to carry the bike on your shoulder. A slightly lower or higher BB height would probably work well for either - it is the ratio of total height vs top tube (so height vs length of the frame) that really defines cross vs gravel. Cross bikes will be taller and shorter (length wise) to have shorter turning radiuses and slightly lower weight at the expense of long term rider comfort and stability at speed. Gravel bikes will be longer and have worse turning radiuses but slightly better stability at speed. You can see the same thing with cars. A shorter bike (just like a short wheelbase vehicle) will also have better approach angles relative to its height. That being said depending on your leg length to upper body torso length you can of course size a cyclocross bike up or a gravel bike down and achieve sort of the same effect. In my opinion these types of geometry questions do not come up nearly enough in relation to the rider, however, as someone with a very long torso is going to be quite uncomfortable with a bike with a short top tube even if the "size" of the bike is correct for their leg length / height. And of course the opposite can also be true. Ideally we should be sizing and matching frame geometry to the persons geometry regardless of the intended use case of the bike.