The posts above are correct!! Gravel in the tank is very dangerous.
After you've removed the gravel, keep a close eye on your turtle and make sure he's pooping normally. It can take months for the gravel to move through his system, so you have to keep watching for a long time. If he's not pooping, then he has to get to a vet right away. The vet can administer laxitives that may help a turtle through a crisis.
Also, as he poops the gravel out, be sure and scoop it up from the bottom of the tank or the turtle will eat it again. Like I said, this can go on for months.