I know this is an old topic but since I haven't found a solution to this problem in the forums i guess it would be ok to post my findings on how to add custom menu items to the context menu.
The solution is rather simple you just have to overwrite the getMenu() method in the GanttEvent. You then just return a menu with the items that you want to add to the context menu.
Here is an example of how this can be done:
class CustomGanttEvent extends GanttEvent
public Menu getMenu()
//Create the menu
final Menu menu = new Menu( getParentComposite() );
//Create the menu items you want to add
final MenuItem menuItem = new MenuItem( menu, SWT.PUSH )