Ooooo, ruby sharks don't normally eat gravel (in fact I think very few fish do, and those that do "eat" gravel usually are sand shifters that blow the sand out their gill slits and don't literally ingest the sand. Saltwater parrotfish are the only fish I know of that literally will eat rock). Pot-bellies on fish can mean one of four things:
1) You've a female that is producing roe (eggs)
2) You've fed the fish until he is nice fat and round
3) Disease such as dropsy or parasites
4) Your fish did eat something that is now impacted in its intestines, preventing the passage of food
Check your water quality and make sure it is top notch. If the fish is still eating and behaving normally, then keep an eye on it and make sure it continues to do so. Sometimes females may get egg-bound (the eggs stuck inside of her) and that can cause their demise, but most of the time the female will either re-absorb her eggs, or spawn regardless of the presence of a male. If the fish is not back to normal within a week, it could be egg-bound.
In the case of disease, parasites, or intestinal blockage, there is not much you can do. Watch for depressed behavior. Is the fish sitting on the bottom? Does it hide a lot? Is it gasping at the surface? Do other fish pick and chase at it? Did it stop eating? Does it look like a pine-cone with all the scales sticking out? Without some of these clues there really is no way to diagnose what disease and to offer any medical advice.
~~Colesea