Wiki has descriptions for the surface types, https://wiki.openstreetmap.org/wiki/Key:surface gravel seems to be general term that includes compacted gravel, I had to look up difference between dirt, ground and unpaved without text descriptions from the wiki that would have been impossible.

I have added some nearby forest paths to OSM and added some bicycle paths alongside roads which were already mapped, using OSM mostly for outside of road network since other maps do not show forest paths and the like at all while OSM has decent coverage