Most new gravel bikes are kind of a cross between a road bike and a rigid 1990s mountain bike... drop handlebars and room for (what are now considered) narrowish off-road tires ~2"+-.
They also generally have modern technology such as hydraulic discs and often 1X drivetrains with a monster sized cassette, although some have 2X drivetrains.
Anyhow, that's what you get if you are in a bike shop looking to buy a new 'gravel bike'.
If you were to find a commonly ridden piece of gravel road and survey what types of bikes people are actually using as 'gravel bikes' then there is no one clear answer. My 'gravel bike' is an older road touring bike with medium/wide tires and mini-V brakes. My buddy rides his cyclocross bike with cantilevers. I see rigid mountain bikes and hybrids doing lots of gravel miles too. I notice fewer folks on full suspension bikes, and my personal experience is that fat bikes aren't ideal for long gravel rides, but I'm sure loads those bikes are used too.
Some people obsess over what are the ideal wheel sizes and handlebar shapes, but everybody has a different opinion, and if they are actually putting their opinion to use and riding then their opinion is correct.