Its such a wide ranging term that you could just ride a road bike with 20mm tubulars and say 'its my gravel bike!'. So basically anything I say, people could disagree with due to bias, educated opinion, or personal preference.
With that said...

- Gearing that is wider ranging than basically anything called a road bike in the 70s thru 00s. Yes, a touring triple or 80s/90s MTB triple would get you the wide range gearing, but those arent road bikes and you need a triple when gravel bikes are quite firmly in the 1x or 2x drivetrain world now, and trending more towards 1x by the year.
- BB drop that is 70 or more, and preferably more, since sitting in the bike vs atop the bike helps with stability and confidence on loose gravel roads at higher speeds.
- HTA and fork rake that result in trail which is more than most road bikes from the 70s thru 00s and less than modern MTBs. More trail than most road bikes helps maintain a straight line on loose surfaces at higher speed. Less trail than modern MTBs helps reduce wheel flop at slower speeds when climbing.


While those 3 things, in addition to tire clearance, are not firm requirements, I do think the 4 combine to represent an ideal gravel bike when compared to many older options. All other differences are really just preference for varying reasons- disc brakes, mounting points, tubeless wheels, flared bars, chainstay length, shifting style, etc.
Many of these options make or a better gravel bike, in my experience, but I fully recognize they are not required and are just preferences.