In a way, bicycles are similar to surfboards. You can have one bike that does everything mostly ok or you can have many boards, each uniquely built for a set of conditions, and so forth.
To me a hybrid bike is a hardtail MTB crossed with a road bike with flat bars and plump tires.....sort of a commuter bike where you need the heads-up aspect to aid in survivability in traffic over the aerodynamic aspects of a road bike. Chuck on curly bars, and now it is gravel bike (but maybe the geometry isn't great for long hauls), though I am partially joking because I think more goes into it that just handlebars.
Anyway, I think the term 'gravel bike' is a marketing term when really, at least to me, it seems to harken back to the the day when road bikes weren't as focused on speed and aerodynamics like they seem to now. Having a bike that wins time trials on pavement on skinny tires is not conducive to multi-surface conditions or traffic where a little beefier frame, more relaxed geometry (better geometry?), and plumper tires makes for more utility. But we need words to describe things, so, gravel bike it is....though I do like the other marketing term "adventure bike" (in contrast to touring bike).
Recommendations? How much $ you got and how much are you really going to ride?
PS
Not a recommendation, but this small shop in Marin does a decent job at discussing his frames/philosophy.
Road + and
Monstercross.