did you get the answer you needed on the other thread? https://es.infragistics.com/community/forums/f/ultimate-ui-for-windows-forms/66927/ultra-win-grid---group-by-column-header#66927
the ID isn't available in this context, but you can use these two bits of information to find which GanttItem was clicked.
DataRow will be the index of the GanttSeries
DataColumn will be the absolute index of the GanttItem (cumulative for all series)