The one-line answer
Gravel bike = the do-everything bike. Road bike = pavement specialist. Mountain bike = trail specialist. If you're not sure, gravel wins.
When a road bike is the right pick
You live in a city with great bike infrastructure, you'll only ride pavement, and you want to keep up with fast group rides. Otherwise, gravel geometry is more comfortable and safer for most beginners.
When a mountain bike is the right pick
You want to ride actual singletrack with roots, rocks, and technical descents. A mountain bike is heavier and slower on gravel roads — you'll outgrow it if flat gravel is your goal.
Why gravel wins for most beginners
Drop bars for hand positions, wider tires for comfort, disc brakes for wet-weather safety, and clearance to grow into bigger rides. It's the bike you'll still love in three years.