What are the main differences between a road bike and a gravel bike, and can a road bike be easily converted?

Are there structural and frame differences or does it just need new tires and lower gearing?

Can a 2013 Cannondale synapse with rim brakes be converted to a gravel bike easily?

Why are gravel bikes like the Ozark G1 explorer 30 lbs dry. But the Cannondale is 20. Are the gravel frames just strengthened to carry racks and bags? Are road bikes unsuitable for bike packing?