We should subscribe to the CollectionChanged event of the ExpandCollapseService.HierarchicalCollectionAdapter of the RadGanttView in order to detect when a GanttTask has been expanded/collapsed:

 this.gantt.ExpandCollapseService.HierarchicalCollectionAdapter.CollectionChanged += HierarchicalCollectionAdapter_CollectionChanged;

 private void HierarchicalCollectionAdapter_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
 {
 //implement logic here
 }