Goldfish cannot eat gravel. If they swallow gravel they will die!
Pellet food is much better than flakes, and usually more nutricous.
Go for a high protein, sinkable (or absorbant) pellet such as Pro Gold
etc.

Is this true?

They arent herbivores, if that is what you are wondering. Sinking food is
better then floaters for goldfish as there is less chance of them sucking
air.

jc