I would feed him in a seperate container for a few days to see if it passes. They will usually go to bathroom right after they eat. Also make the feeding conatiner water slighty warmer than his tank water. I have had turtles eat gravel and they passed it. If you dont see it in his poo, a vet checkup cant hurt. My guess would be that this is not the first time hes eatan gravel. Its just the first time you have seen him do it. Taking him off the gravel is a good idea. If you want something on the bottom as substrate you could use playsand. I think its very attractive, especially with DBTS.
Chris
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Hey whats up, I'm Chris and I currently have 1.0 Softy, 0.0.1 Midland Painted, 1.0 Bearded Dragon