public class GeoprocessingOptions 

{
  //These options are for behavior of interactive GP tools _only_.

  var overwrite_gp = ApplicationOptions.GeoprocessingOptions.OverwriteExistingDatasets;
  var remove_gp = ApplicationOptions.GeoprocessingOptions.RemoveOverwrittenLayers;
  var addoutput_gp = ApplicationOptions.GeoprocessingOptions.AddOutputDatasetsToOpenMap;
  var history_gp = ApplicationOptions.GeoprocessingOptions.WriteGPOperationsToHistory;
}

{
  //Note: changing these options modifies behavior of interactive GP tools _only_.
  //Use the ArcGIS.Desktop.Core.Geoprocessing.GPExecuteToolFlags enum parameter
  //to modify the behavior of Geoprocessing.ExecuteToolAsync(...)

  //Note: Needs QueuedTask to run
  {
    ApplicationOptions.GeoprocessingOptions.SetOverwriteExistingDatasets(true);
    ApplicationOptions.GeoprocessingOptions.SetRemoveOverwrittenLayers(false);
    ApplicationOptions.GeoprocessingOptions.SetAddOutputDatasetsToOpenMap(true);
    ApplicationOptions.GeoprocessingOptions.SetWriteGPOperationsToHistory(false);
  }
}