Gravel bikes run the gamut from mountain bike-like bikepacking bikes to road-bike like racy setups. You can pick what style of bike best fits the riding you want to do. There are hundreds of options. For regular rail trail / crushed gravel riding, pretty much anything will do. In general, I'd go with discs and room for at least 45s. Everything else is debatable.
Contrary to the posters above, I would not recommend a traditional CX racing bike for most people who want to do gravel events or ride lots of fire roads / farm roads. Traditional CX bikes are designed for 1 hour all-out racing efforts on slick courses, not for the longer, slower riding most people riding gravel are doing. Racing CX bikes typically have aggressive handling (more aggressive than even road racing bikes to handle tight CX turns), very short wheelbases, high bottom brackets and very short head tubes that result in large saddle-to-bar drops. All of this results in a very agile and responsive bike that can also be both twitchy and uncomfortable. Also, CX-specific gearing is often too tightly grouped for hilly gravel riding and CX tires are not very good for riding gravel as the tread is often too heavy.
Of course, many manufacturers are now offering longer / slacker bikes that can fit wide tires and calling them "CX bikes", so the traditional template isn't universally true. In general, ignore the labeling of the bike ("gravel racing", "adventure", "bikepacking", "cross", "road plus" etc) and focus on the geometry and features.
Contrary to the posters above, I would not recommend a traditional CX racing bike for most people who want to do gravel events or ride lots of fire roads / farm roads. Traditional CX bikes are designed for 1 hour all-out racing efforts on slick courses, not for the longer, slower riding most people riding gravel are doing. Racing CX bikes typically have aggressive handling (more aggressive than even road racing bikes to handle tight CX turns), very short wheelbases, high bottom brackets and very short head tubes that result in large saddle-to-bar drops. All of this results in a very agile and responsive bike that can also be both twitchy and uncomfortable. Also, CX-specific gearing is often too tightly grouped for hilly gravel riding and CX tires are not very good for riding gravel as the tread is often too heavy.
Of course, many manufacturers are now offering longer / slacker bikes that can fit wide tires and calling them "CX bikes", so the traditional template isn't universally true. In general, ignore the labeling of the bike ("gravel racing", "adventure", "bikepacking", "cross", "road plus" etc) and focus on the geometry and features.