As said, check that on the inside of the rotor (towards engine) there should be a backing plate that "visually hides) the rotor, protecting it from gravel and other debris making it's way between rotor and caliper on the inside. Is the gravel making it's way between rotor and caliper on the outside of the rotor? The wheel should keep this from happening. Is the rotor getting grooves getting ground on both inside and outside of the rotor?