You can use the Path property of your DirectoryEntry object.
DirectoryEntry user = null;
DirectoryEntry gcEntry = new DirectoryEntry("GC:");
IEnumerator ie = gcEntry.Children.GetEnumerator();
while (ie.MoveNext())
{
gcEntry = (DirectoryEntry)ie.Current;
}
using (DirectorySearcher searcher = new DirectorySearcher(gcEntry))
{
searcher.Filter = string.Format(ldapQueryString);
SearchResult result = searcher.FindOne();
string[] pathParts = user.Path.Split(new char[] { ',', ' ' });
}