I would visit this page
Gimp Chat Tutorials Index and scroll down to Draconian Metal Wurx Series,
The tutorials of interest would be
MW#17 - Metallic Control Knob
MW#19a - Indicator Lights - Part A (PDF)
As both show how to make a metallic outline that can have an orb inserted into them instead of the tutorial requirements but worth reading through the others as well.
As to a glossy orb tutorial, I would recommend my own tutorial
Glossy Ball Tutorial although an improved xcf of this can be downloaded from
HEREor this tutorial
Ron Scott's Tutorial - Glassy SpheresMy avatar was made by combing both of these techniques for base and orb.