Given the XML based file system used by GanttProject (see previous post), I have modified the XML file generated by the PM Portal tool to support GanttProject. You can now create an export from the PM Portal and open the file in GanttProject. The big advantage of this new functionality is the ability to generate reports in HTML, RTF, and PDF formats.
I am evaluating a bi-directional interface, but for now it is one way only. Give it a try and let me know what you think in the PMPortal Discussion forum.