Step 7:
make a copy of the line layer. (this is important)
select the copy layer.
Use "Script-Fu"->"Alpha To Logo"->"Glowing Hot"
Effect Size 20 pixel, black background.
Remove the new background layer and move the line layer to to top.
Hide the layer with the cross and save as a png image.
Save it as gimp native, too. you might need that again.

To have the gauge show something, a pointer is needed.
This pointer will be created by a C program. The howto for this program will be in the C-Programming section soon.