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
  }
}