What is a gravel bike? Knobby tires on a road bike? How is that different than a cyclocross bike?
It's a very clever marketing strategy. The only thing that a manufacturer has to do is to add some extra tire clearance, tweak the geometry a little bit and sell it at an inflated price....I went the cheap route and converted two of my rigid forked mountain bikes into a gravel bike. All I did was to put drop bars on it and I got myself a very capable gravel bike that handles great on pavement and off road.