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

{
  //Gets a specific GeoprocessingProjectItem in the current project
  // example: "myToolbox" is the name of the GeoprocessingProjectItem
  GeoprocessingProjectItem gpProjItem = Project.Current.GetItems<GeoprocessingProjectItem>().FirstOrDefault(item => item.Name.Equals("myToolbox"));
  // Use gpProjItem;
}

{
  //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(";", [toolItem.Path, toolItem.Name]);
      // do something with the newTool
      // for example, add to a list to track or use them later
    }
  }
}