I'm probably showing my ignorance here, but what exactly is the definition of "a gravel bike".
I think it's something between a touring bike and a mountain bike, but I really have no idea.
It’s basically a road bike, with gearing, frame design, geometry, tyre clearances, that allows it to be used on uneven / loose / rough surfaces, without compromising ride quality too much. Think of them as cyclocross bikes that have been ‘softened’ to give them broader appeal. There are certain things that typically mark a gravel bike out. Higher than typically found on a road bike bottom bracket position, shorter chain stays, disc brakes, clearance for 35+mm tyres, big cassettes, rear mechs to cope with them, and so on and so forth.