TMonk

^
Can't tell if that was a serious or troll question, but you got me to bite either way.

Cyclocross is a high-intensity racing discipline featuring short (typically ~60 min) races with obstacles, mandatory run-ups and other interesting features. To accommodate this the bikes have aggressive geo, lightweight frames, high BB's and moderate tire clearance. Many high end cross race bikes don't even have bottle cage mounts - you don't want bottles on your bike when you're shouldering it multiple times per lap.

Gravel riding is mixed-surface and typically long in duration, often categorized as "epic" and a number of other trendy words these days. To accommodate this the bikes have relaxed geo, lower BB's, room for bigger tires, multiple bottle cages and is generally more "ovebuilt".

Of course you can use a cross bike for gravel applications or a gravel bike in cross races, but the deeper you get into it the more you might appreciate a purpose built machine. Many (most) consumers fall somewhere between that dichotomy and as such that are plenty of off-the-shelf commercial "cross bikes" that are well suited for both. Or you could just buy a gravel bike - unless you want to get way into cross, that is.