Currently it is not possible to customize the content of the built-in Gantt editing Dialog, and the only option is creating a custom Dialog, and opening it by handling other Gantt events like taskClick/taskDblClick.

Adding a Dialog template that will be rendered within the built-in editing Dialog would be a nice enhancement enabling developers to have their custom content while still keeping the built-in Gantt editing mechanics and events.

For further fine-grained tuning, separate templates for the content of each TabStrip tab (General, Predecessors, Successors) would also prove useful.