you should read up on how to customize nuke.
in a nutshell, all the files you want to use (gizmos + icons) should be living in nuke's plugin path.
then there are python commands you can write into a file called menu.py which also has to live in your plugin path:
toolbar = nuke.toolbar('Nodes')
toolbar.addCommand('Image/My Gizmo', 'nuke.nodes.MyGizmo()')
the above will add a gizmo called MyGizmo.gizmo into the existing Nodes toolbar under the Image menu and call the entry "My Gizmo".
Using the same way you can add stuff to the right click menus in the DAG, panels or viewer.
try typing the following into the script editor:
help(nuke.menu)
hope this helps.
frank