To me, the difference between a gravel bike and a cyclocross bike is 99% marketing. They are defined by their target markets. Cyclocross bikes have tubular tires and are more aimed at racing cyclists. Gravel bikes are more targeted at your average weekend warrior, normally have clinchers, and probably have slightly fatter tires.
Sort of reminds me of the 90s when there were like 50 different genres of techno music, yet it all sounded pretty much the same.
BITD a cyclocross bike was pretty much a high quality touring bike with a slightly higher bottom bracket position, and knobby sew ups of course.
A PX 10 from 1972 or earlier makes a good gravel grinder. Later ones had much steeper and tighter geometry.