I erroneously believed that hovering over a gant item gave all the columns data, instead it is simply the Title, Start and End. Is there anyway to get more information on popup from the hover action?
The ToolTip of the task could be easily modified by changing the ToolTipTemplate of the EventContainer using implicit styles. I've prepared and attached a sample project which creates a custom gantt task with custom property which value is shown in the ToolTip of the task.