I certainly dont think of hardpack limestone rail trails as gravel, but to each their own. Most every hardpack limestone rail trail or river bottom trail I have ridden has a well groomed hardpack surface, very limited elevation change(thats a feature of river and rail trails), and can be comfortably ridden on 30-35mm tires for most of the riding season, when dry.
Routes like these can be great fun, but to me they seem very different from gravel road riding and what this Ozark bike seems to have been designed for. Obviously the Ozark bike can then handle hardpack limestone trails, but I think its a bit different to build an older road bike with some 32mm tires fpr $250 and say 'look, I built a bike that competes with the Ozark!'...assuming the Ozark ends up being more than just a BSO.