What is a gravel bike?
Andy asks a very good question here!
For my (somewhat cynical) money, much of the "push" for gravel bikes as a niche was initially for product differentiation and a way to sell more bikes, as with other increasingly thin slices of the pie to goose sales when the market goes a bit flat. Cynicism aside, a drop-bar road bike that has greater versatility to handle a wider variety of surfaces turns out to have a lot of appeal to many people and may come closer to being a "quiver-killer"/all-'rounder than other kinds of bikes currently on the market. I predict gravel bikes will become more specialized as have other niche-bikes. It won't be too long before we see more widespread use of front or front/rear suspension and then we'll have something akin to a full-sus 26in/650B MTB but with fat 700C tires and road/flared drop handlebars. Several makers have recently floated bikes with just that configuration, well received in recent cycling press.
All of my bikes (one from 1938, but mostly made 1970-2012) have been ridden on gravel, from back in late 1970s/early '80s America when tire sizing and marketing was driven by advertised weight. Some of the Specialized brand tires I rode at that time measured 1-2 (later) ETRTO sizes narrower than marked. Tires much arrower than labeled meant an automatic reduction in weight if you could believe the label and many did, thanks to marketing. I used them anyway, as there was a shortage of quality touring tires available in my area at a time when special-ordered Wolbers in true 27 x 1-1/4 could easily take 6-8 weeks' time to arrive in my locale.
A lot of my bikes are "old/er", made when more generous clearances were common, so really fit the more modern general ideal for gravel bikes: A road-bike frame with more relaxed road-bike geometry and larger 700C wheels shod with wider tires, typically starting at about 34mm and going upward (mine range from 32mm to 38mm and these have worked well for me on most gravel). If you find an older frame designed around 27in wheels, you can gain 4mm in tire/mudguard clearance by converting to 700C and longer-reach brakes.
A good question to also ask is "what kind of gravel?", for it ranges from what I regard as benign (finer than pea-gravel, tamped well into car tire-track ruts) to malign (fresh-pour in what we here call "three-quarter minus" to full-on ballast). For the latter, I prefer my bikes built with clearances to accommodate my 26x2.0 Schwalbe Duremes, as on my Nomad, tandem, and repurposed MTB née Enduro-Allroad (1987 Diamondback Transporter). While all my bikes "will" traverse gravel of one sort or another (including my 1970 Windsor Professional-based fixie on 25mm Continental road slicks), I am most "comfortable" doing so on those with wider tires. A fave for the more benign sort of gravel is my 41 year-old tourer, closest to Thorn's old Club Tour. It will happily accommodate 38mm tires with adequate, safe clearance for mudguards and run rackless and with short-reach/shallow-drop handlebars, gives a good impression of most modern steel-based gravel bikes. For the bad stuff, the lighter deraileur Transporter does as well as the Nomad for unladen or lighter loads when the advantages of a Rohloff drivetrain and/or expedition-grade carrying capacity is not essential.
I'll be a heretic here and suggest -- based on my own experience -- buying an older road bike with generous clearance for wider tires is a good way to test the waters before splashing out for an all-new bike. A lot of fun can be had at low cost while you learn and refine priorities and preferences on a budget now such bikes are going for peanuts at yard and boot sales. My 2012 Nomad is my latest bike, but might not have been had I not learned what I needed for a specific task -- extended, self-supported long-distance touring with expedition loads on the roughest of roads and tracks. It is also possible to retrofit an older bike with a Rohloff drivetrain, then swap components over if you later find a frame that better suits your needs.
Just some thoughts to ponder.
Best, Dan (...who always looks forward to members' "New Bike Day")