public interface IGPHistoryItem 

string openProjectPath = @"D\DATA\IGPHistoryItemTestProject\IGPHistoryItemTestProject.aprx";
await Project.OpenAsync(openProjectPath);
MapProjectItem mapProjItem = Project.Current.GetItems<MapProjectItem>().FirstOrDefault(item => item.Name.Equals("Map", StringComparison.CurrentCultureIgnoreCase));

var map = await QueuedTask.Run(() => mapProjItem.GetMap());
var ftrLayer = map.Layers[0] as FeatureLayer;
string tool1 = "management.GetCount";
var args1 = Geoprocessing.MakeValueArray(ftrLayer);
var env = Geoprocessing.MakeEnvironmentArray(overwriteoutput: true);
GPExecuteToolFlags executeFlags = GPExecuteToolFlags.AddToHistory;
var t = await Geoprocessing.ExecuteToolAsync(tool1, args1, env, null, null, executeFlags);

IEnumerable<IGPHistoryItem> hisItems = Project.Current.GetProjectItemContainer(Geoprocessing.HistoryContainerKey) as IEnumerable<IGPHistoryItem>;

String hitemID = "";
String hitemToolPath = "";
IGPResult hitemGPResult = null;
DateTime hitemTimeStamp;

foreach (var hitem in hisItems)
{
  // common IGPHistoryItem and Item properties
  hitemID = (hitem as Item).ID;
  hitemToolPath = hitem.ToolPath;
  hitemGPResult = hitem.GPResult;
  hitemTimeStamp = hitem.TimeStamp;
}