I probably should have been clearer. I can distinctly recall a time in the mid 20Teens, before anyone made up the word monstercross (that I am aware off), where it seemed gravel bikes were just CX legal bikes with relaxed road geometry (Original Diverge was Roubaix geometry with space for 35s, and the Focus Paralane was actually an endurance geo road bike with space for 35s). In fact, as I recall, squeezing a 35 into a "gravel" frame was about as big as one could go on any of those early "gravel" specified frames. The influence of a few frame builder (brands, really, from the US) pushed gravel into the wide tire realm and they subsumed the "monstercross" wide tire builds as well as the mullet 1x drivetrains.
Of course, CX would stay "defined", because of the UCI. I probably shouldn't have brought up the CX discipline/class, except to say that the current gravel craze started on CX bikes, but it didn't stay on them for very long, because CX geometry os pretty aggressive and not conducive to the kinds of riding the gravel culture embraced.