Unpaved roads vary across the US. In a vast amount of rural America "gravel roads" are well understood to mean the local mix of rock and dirt that can be compacted into a roadbed. Where I lived in VA that meant a fairly hard clay with comparatively little stone compared to say the roads in a more mountainous area. I've lived on gravel roads much of my life and live on one now.
The only thing that gravel roads share in common is that they are inconsistent. Which is what makes them so much fun to ride.