Post #2 is my story as well.
Originally, gravel bikes were intentionally designed not to be overly-differentiated, so that they could handle all road surfaces and used in exactly the way the OP describes.
Even the more differentiated, slack-geometry gravel bikes should be fine on-road with some wide but slick low-rolling resistance (eg Rene Herse) tires.
The roads I ride here encourage me to have at least 38mm tires, as they are in such poor condition.