I do not think there is a good definition of a gravel bike, but as a group from what I have seen they generally have a tire size up to about 45mm, shorter chainstays than a touring bike, typically a 1X drivetrain, drop bars which may or may not have the drops wider than the tops of the bars, and an endurance posture for the rider. I do not think I have seen any that were 650b, all the bikes I can think of that I have seen labeled as a gravel bike are 700c.
Or to summarize, generically I think of a gravel bike as a road bike with wider tires, a 1X drive train, and a more comfortable posture for longer rides.
First photo. I think my titanium touring bike would be close to a gravel bike, except the chainstays are too long and the drive train is a 3X. I have 37mm tires and 45mm wide fenders fitted to it that would have to come off if I wanted to call it a gravel bike. And of course the racks would have to come off it. Photo is from my bike tour this past June.
Second photo. My Nomad Mk II has the right handlebars for a gravel bike, but the bike is not as close to a gravel bike as my titanium bike. Photo is during winter a few years ago. At the time of the photo I had 50mm wide Marathon Winter tires fitted, but I usually have 57mm wide tire on it. I should note here that since the Nomad Mk II was sold as a flat bar bike and I have drop bars on mine, mine is a 590M size since the Nomad Mk II was not available in the S frame sizes. I built mine up from the frame, not purchased as a complete bike. That is the size I needed to get the correct reach for my body size, thus the really tall seatpost.