It's going to be much smoother than the beltgrind. The southeast section of the beltline dirt/gravel is not typical of gravel roads in georgia as that section has significant bumps and over-sized washboarding from the heavy vehicles used to remove the tracks and ties. The other sections with railroad ballast or sidetrail are very different as well. Basically only comparable part of the beltgrind might be the super smooth but mildly sandy section North of the horse track checkpoint.
State of gravel is going to be on dirt roads that cars and trucks can comfortably drive at 30+ miles an hour. Should be about perfect for bikes at 12-17+ mph