I think the difference is mostly marketing horse**** intended to sell more bikes. Most people would do fine on either.
When you actually compare the geometry of gravel and cross bikes the differences in mm are quite small. The biggest noticeable difference I suspect is from tires. I have two cross bikes a Boardman CXR and an older S-Works Tri-Cross. The Tri-Cross is probably closer to a modern gravel bike and accepts bigger tires. They're both fine on gravel. If I need 2" tires I prefer to use a MTB with proper suspension.
If I look at the head tube angle for example a large CXR (cross) is the same as a large Open UP (Gravel)