You can contribute to GanttProject by fixing bugs, enhancing existing or adding new features. If you decide to code, we appreciate that but we have to tell that this requires pretty good knowledge of Java/Kotlin and software engineering practices. You probably don't want to hack GanttProject for educational purposes.
If you're up for writing some Java/Kotlin code in GanttProject,we kindly ask you to read the guidelines, take them seriously and follow them even if you disagree with any particular point. The guidelines help us to keep the code base healthy and keep moving on.