Keep it damp in a lightly covered container where there is some air exchange, but not so much that the soil dries out. Bacteria, fungi, and all the other soil critters break down organic matter during the composting process. The easy-to break down organic matter is consumed first. then the more difficult types are consumed, but very slowly. It takes many years for all the organic matter to be consumed. The purpose of composting is to get rid of the easily decomposable organic matter. The rest of the OM is not likely to cause any problems when the compost is used as a subatrate under the gravel. No heavy bubbling, or black deposits of iron sulfide, or bacterial clouds above the surface.