In the same way as MTB has experienced an evolution to new formats to adapt the bikes in a better way to their actual uses, something similar has happened in the road bikes.
More and more riders ask themselves why do they have to give up riding on the gravel and at the same time want to stay fast no matter if it is on the road or the path. Gravel philosophy is the one of those not aiming to break any record; instead, they prefer enjoying a wider use range for their bikes. How many times have we discovered a tempting path while riding on our road bikes?
Gravel bikes set their focus on comfort and being able to face large journeys with pannier racks and all our luggage on. They defend a return to the roots of cyclism: enjoying the ride. Our bike must represent no problem to face any possible adventure, no matter if it is on and off the road, without excessive ballast or impossible positioning.
When thinking about gravel bikes off the road, the first thing we think about is cyclocross bikes. What is the difference between them?
The main difference between both concepts is that Cx bicycles are race-oriented. Gravel bikes, on the other hand, have a more relaxed concept, rack mounts are included, they are designed to ride many hours in a row, their bottom bracket height is lower as there will be less obstacules to care about. The entire geometry concept targets higher comfort levels. Cx bikes target sprint races no longer than two hours, maximizing performance and ready to go through tricky trails and handle with big amounts of mud.
On the other hand, gravel bikes aim to offer a high comfort riding level so that riders can face long distances without trouble. But then, what is the difference between endurance or gran fondo and gravel bikes?
As commented above, gravel bikes are mainly focused on adventure and long journeys. In that point where kilometers start to hurt, gravel and endurance bikes seem to be similar.
In that case, they comparison is only possible while riding on asphalt, as endurance are nothing but road bikes with an specific non-racing geometry, relaxed riding positioning and technical solutions to filter vibrations. Other solutions present for long rides are wider tires: endurance bikes feature up to 28c but that is nothing compared to 40c of gravel bikes. Both usually use disc brakes so, as it happens when comparing to Cx bikes, they have similar aesthetics.