GanttProject code is covered by a number of different licenses, mostly GPL. Here is a brief summary of what you can do and what you can't

• You are allowed to use unmodified GanttProject binaries for any purposes, including commercial usage and reselling, at no charge, royalty fees, etc.
• You are allowed to modify the sources of GanttProject
• If you redistribute modified binaries of GanttProject, you must provide all the sources to the receiver of a binary copy on his first request. Modifications should be covered by the same license as original sources
• If you don't make your modifications publicly available, you don't have to publish the modified sources

That is, everybody who gets modified GanttProject binaries from you should be able to get the sources under the same license.

Please note that some of the libraries used in GanttProject are covered by Apache license. There are different opinions about compatibility of Apache license and GPL. Free Software Foundation believes that code under Apache license should not be used in GPL software. On the other side, Apache Software Foundation thinksthat it is their right to decide who may use their code and who may not. Feel free to take any position is this holy war and let us just write the code. If you care about all those bells and whistles of GPL, don't use GanttProject. There are many nice competing programs around. As an option, you may contact us and ask to give you the code under some other license.

... and yes, as a third option you may go ahead and implement those libraries under (L)GPL and generously share your implementation with the world for free. Good luck!