Originally Posted by msu2001la

I don't think you're wrong in thinking that, but it's also a broad generalization. There are definitely some race oriented "gravel bikes" that have very similar tire clearance and geometry as some CX race bikes. There are also some more casual recreational CX bikes that would fit well into a "gravel bike" category that didn't exist a few years ago. Also, using this same generalization, CX bikes are really just more comfortable road bikes with more tire clearance and less aggressive geometry, etc.

Road Race - Road Endurance - Road Touring
CX Race - Gravel Race - Gravel Adventure

Now that disc brakes are everywhere, the differences between these bikes really comes down to frame geometry, tire clearance and gearing. To take it a step further, the differences between gravel adventure bikes and XC mountain bikes are getting pretty blurry too. There are several gravel bikes out there that are essentially hard tail XC mountain bikes with drop bars and rigid forks, running MTB rear components and hub spacing. Several manufactures are making gravel bikes with suspensions as well.

I think the bigger question I have is "what attributes make a good gravel bike?". This involves answering an even bigger question of defining what "gravel riding" even is. Is it flat and fast racing on gravel roads across open plains? Is it climbing a rugged two track twisting up into the mountains where you want to bring lunch and a few beers? Is it cross-country traversing over cobbled farm roads in northern France? Is it cruising a twisty singletrack trail along the shore? Each of these situations will favor a slightly different type of bike, but does that matter? Unless you're racing, it seems like your style of riding would dictate which type of bike works best for you, similar to deciding between road race and endurance styles of bikes.

Of course the bike industry is interested in selling more bikes, but also there's only a small segment of people who feel the need to own a specific bike for every purpose. I think there are just as many people out there that are finding they can use a single bike for all of those situations, plus more. Unless you're really into fast group rides or road racing, a CX or gravel bike with a second set of wheels makes a pretty good road bike for solo riding, touring, commuting, occasional CX racing, etc. Heck, there's a guy in my local CX series that is on the podium every weekend in the masters series while riding a flat bar rigid fork XC mountain bike.