Szaki wrote:

They live to 8-9 years.

Depends what type, commons etc can live upto 20 years. I believe the
oldest one recorded was 43 years old.

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.