GanttProject will be covered by GPLv3 starting from version 2.5. We made this update from GPLv2 mostly for the sake of license compliance with the libraries which we use. For most GanttProject users this requires absolutely no action. GanttProject remains both gratis and libre, you can use it for any purposes, including commercial, given that they are legal per se. Only those who link code of their public proprietary web applications with GanttProject code should be worried, but I really doubt that we have such users.