public sealed class GPExecuteToolEvent : ArcGIS.Core.Events.CompositePresentationEvent<GPExecuteToolEventArgs> 

ArcGIS.Desktop.Core.Events.GPExecuteToolEvent.Subscribe(e =>
      {
          string id = e.ID;                   // Same as history ID
          if (e.IsStarting == false)  // Execute completed
              _ = e.GPResult.ReturnValue;
          System.Windows.MessageBox.Show("event triggered.");
      });
await Geoprocessing.ExecuteToolAsync("management.GetCount", Geoprocessing.MakeValueArray(@"c:\shape_file.shp"));