I think the one distinguishing feature of a gravel bike is that it resembles a canonical drop-bar road bike, but can accept wider tires (say 35-38mm) that make off-pavement riding more pleasant (or possible).
For example, the current Trek Domane would qualify. There are of course many purpose-built gravel bikes, but they might be less capable on paved roads.
Attributes like disc brakes and lower gearing really help, but I wouldn't consider them defining characteristics (but would insist on them for any of the off-road rides I do). Disc brakes also allow fitting 650b wheels with wider tires.
It should be able to handle most non-technical single track and very rough, steep, unpaved fire-roads to get the full potential, while ideally not presenting a huge on-pavement penalty.