There are different types of gravel and different gradings of it. Smooth river rock that is classified (sieved for size) is different than crushed rock or granite or sandstone or other rock types. Some gravel is smooth, and some has sharp enough edges to cut tires. So I think you might need to clarify the gravel type and then ask the question.

Apart from this input, I will defer to the more experienced long distance riders. Hope this helps a little.