Eric F 

At a very basic level, a "gravel bike" is usually a drop bar bike with mid-width, moderately-durable tires. Typically, it's not as efficient on pavement as a pure road bike, and not as capable in the dirt as a MTB when things get steep and technical, but it absolutely excels on dirt roads as long as conditions aren't extreme. That said, there is variation within the range of "gravel bikes". Some are optimized for speed, some are optimized for hauling gear, and many fall somewhere between. A gravel bike tends to have a lower bottom bracket than a road or cyclocross bike for improved stability. Angles may also be a bit slacker, and wheelbase might be longer. Choice of tire tread and width is largely affected by terrain and performance needs, ranging from semi-slick to MTB-type knobs.

For someone who doesn't know what a gravel bike is, you are already spouting strong opinions about them, including a bike that is a better choice. From you, I'm not really surprised.

Yes, you could ride a road bike with wider tires on gravel (some people do that). Yes, you could also ride a MTB (some people do that, too). For me, my gravel bike rides tend to be a mix of pavement, dirt roads, and singltrack. Of the bikes in my stable, none of them handle the combination of surfaces better than my gravel bike, and I pick ride routes that I wouldn't do on any other bike because of how versatile it is.