"Gravel bikes are just cross bikes"... except for geometry, riding position, riding characteristics, tire clearance, gearing and sometimes brakes.
"Gravel bikes are just 90s mountain bikes"... except for wheel size, geometry, ride position, ride characteristics, gearing, brakes and handlebar setup.
"Any bike can be a gravel bike"... except most road bikes aren't quite robust enough (yes, there are exceptions, thus the word "most") for long days with rocks pinging off your downtube and don't have quite enough tire clearance for 45s, most hardtails (especially modern ones, but yes there are exceptions thus the word "most") are not really designed for long miles sitting down with multiple hand positions, cross bikes are often (ditto, except I used the word "often" here) just a bit too racy and have handling that's often just a bit too twitchy for hours on real gravel. If only someone would make a bike suited exactly to riding long gravel roads... like brands build bikes suited exactly for time trials, downhill mountain biking, climbing the Alps in the Tour, fully-loaded touring, BMX racing, cycloball, etc.
Overall, I don't get why so many people find gravel bikes objectionable. They're just more practical and more versatile road bikes. They're suited to their intended purpose. Also, "gravel" is a spectrum from slow riding on rough singletrack to racing on smooth, groomed roads and paths. It's great to have a similar spectrum of options readily available so buyers can get exactly what they want for their local conditions.