No such mapping is maintained by the system. You can use GetNearest to get it. Each PointNode does have a field called 'gameObject' which is a reference to the gameObject that is was created from so you can do
var graph = AstarPath.active.astarData.pointGraph;
var node = graph.nodes.FirstOrDefault(n => n.gameObject == targetObject);
However that would not be particularly performant if you are doing lookups very often.