"Gravel bike " is a very clever marketing strategy. The only thing that the manufacturer has to do is increase tire clearance and tweak the geometry a little bit and then call it a "gravel bike".
The way I look at it is this:
- road bike = Ferrari
- MTB = Jeep
- Gravel bike = AWD Subaru
IOW a gravel bike is a cross over, jack of all trades do it all kind of a bike. It handles good on pavement and can still go off road and have fun on some mild singletrack.