Ah gravel bikes. Most roads have been gravel roads anyway until quite recently, in the grand scheme o things, and for sure anywhere outside a city. For that matter, the course of the Paris Roubaix is probably way more "gravelly" than most people will ever encouter on their purpose built gravel thing.
So in this sense, gravel bikes ride on roads, just a different road surface, and so are road bikes, and lots of road bikes historically could do that because that's the surface that roads had. This would suggest that retro gravely bikes shouldn't be treated too differently. Although, most people ride their mountain bikes on the dirt roads so that would make them road bikes too