Greeting from Syncfusion support.
Currently there is no support to define parent task as predecessor and predecessor to parent task in EJ1 and EJ2 version of Gantt. We are considered this feature as high priority and included in our EJ2 Gantt development roadmap. For this we already logged feature report. We are planned to move Gantt from preview release to main release, so we are already lined up some major features, so we could not start the feature immediately. This feature will be implemented and included in any of our upcoming main volume release. You can track the status of this feature by using following feedback link.
Please get back to us if you require further assistance on this.
Pooja Priya K.