If its gravel then it is quite likely that the turtles will eat it unintentionally. Personally, I would recommend it is all removed as food and waste would probably get trapped between the layer of gravel and the larger stones. Our tanks are mostly "bare bottomed" but large slate pieces and stones do look good.
If you go to the main Shelled section and repost you should get some more answers.
Which turtles is it for and do you need any other info regarding their tank, filter, lights, basking area etc?
Turtle tanks really should not have a lid - don't know if yours does.
Welcome to the Fourm.