public sealed class GeoprocessingProjectItem : ArcGIS.Desktop.Internal.Catalog.ItemInfo, ArcGIS.Desktop.Core.IMetadata, ArcGIS.Desktop.Core.IProjectItem, ArcGIS.Desktop.Core.ISearchableItem, System.ComponentModel.INotifyPropertyChanged  

GeoprocessingProjectItem GPProjItem = Project.Current.GetItems<GeoprocessingProjectItem>().FirstOrDefault(item => item.Name.Equals("myToolbox"));

//Gets all the GeoprocessingProjectItem in the current project
var GPItems = Project.Current.GetItems<GeoprocessingProjectItem>();
foreach (var tbx in GPItems)
{
  //Do Something with the toolbox
}

var gpItems = CoreModule.CurrentProject.Items.OfType<GeoprocessingProjectItem>();

      // go through all the available toolboxes
      foreach (var gpItem in gpItems)
      {
          var itemsInsideToolBox = gpItem.GetItems();

          // then for each toolbox list the tools inside
          foreach (var toolItem in itemsInsideToolBox)
          {
              string newTool = String.Join(";", new string[] { toolItem.Path, toolItem.Name });
              // do something with the newTool
              // for example, add to a list to track or use them later
          }
      }