I recently found a project on source forge which is 100% java based and does a very good job of generating Gantt charts.   It uses an XML based file system to store all information on the project.   It allows you to work on one project at a time, but given the ease of use and extensive reporting features, it is worth a look.

ganttproject.sourceforge.net

Given the XML based file system used by GanttProject, I have modified the XML file generated by my PM Portal tool (currently shelved) to support GanttProject.   I can 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.