From email: laconic one

GanttProject Desktop Support
2

Assuming that you installed GanttProject, not some other software:

  1. Make sure that you have Java Runtime. We recommend using Java 8 from Oracle, however if you have any concerns about the license, you can download Java 8 or Java 11 from other vendors, e.g. from Amazon
  2. If you’re on macOS, you may want to read this knowledge base article
  3. If you’re on Windows, you may want to read this and this