The term which ended up becoming the name of bikes that were not limited to riding on smooth pavement but were designed for any road ended up becoming something rather unfortunate. While I've mentioned this ad nauseam here on my blog, I will risk saying it again- "Gravel Bikes" is absolutely wrong for this genre' of bicycle. It should never have been the term adopted for a bike that is possibly the most versatile style of bike made in the last half century. But there ya go. I cannot change that unfortunate term anymore than you can now. So, please understand that I do not endorse the term 'gravel bike', but for reasons of communicating simply and clearly, that is the term I will use. Okay, with that out of the way.......
As I stated in my BMC review linked above, in the late 00's and early years of the last decade, there were no 'gravel bikes' at all. What we were using were adopted bikes from other cycling genres. Bicycles meant for mountain biking, cyclo cross, randonnuering, and other types of bicycles were all used on gravel roads and still can be. This is very important to understand. Any bicycle you can ride most likely could be ridden on a gravel road, but it may not work all that well, or be satisfying in the end. Nuff said there.....
The point I wanted to make is that early gravel events after 2005 were populated with a cornucopia of bicycle types, but mostly they shook out to be hard tail 29"ers and cyclo cross bikes. Elements of both were necessary to end up with the 'gravel bikes' we all enjoy today. The big tires and more relaxed geometry of 29"ers were matched up with the sensibilities of cyclo cross bikes, namely the drop bars and lighter weight frames, to come up with where we are today. Add a dash of early 20th Century road bike design and you are pretty much there. A bike that can go from pavement to single track with aplomb. Any road available becomes a road you could undertake a cycling adventure on.