Gravel vs Endurance bike for road Gran Fondo?

There seem to be a lot of similarities between a road endurance bike with disk brakes (e.g. Giant Defy, Trek Madone, Specialized Roubaix) and the sportier end of the gravel / adventure / all-road bike spectrum.

If one considers purely road use, would a gravel bike with 28-32mm road tires be at...