The fish is probably just trying to find some food. After all, goldfish are a captively bred carp (bottom feeders). I've seen a few goldfish do this with no problems. It could lead to problems with the swim bladder as well as ripping internal organs if the gravel were to be digested. I usually go with a bare bottom tank, but if I do use a substrate, I use the marble like pebbles. Or you can use bigger rocks.
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Phishie

"An ounce of prevention is worth a pound of cure."