A gravel bike is a bike that you ride on gravel.
Really, though, the one commonality of gravel bikes are wide tires. Everything else is optional, and I'd dare say depends on where you're riding gravel at. Here in the PNW we have miles and miles of forestry roads, which are often steep and twisty, so powerful disk brakes are helpful.If it's wet, fenders with lots of clearance make sense.
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If someone tells you that you have enough bicycles and you don't need any more, stop talking to them. You don't need that kind of negativity in your life.