With a little further stop-moving, I think I have the one I want.

Before you finish, make your last gradient color white, and move your stops until your drawing looks like the rainbow you want. The bottom of your L-shape should be at the bottom of your semicircle. Double click on the L-shape to insert and edit several stops, choosing your desired rainbow colors at each stop. You should click on the gradient tool as well to get the L-shaped indicator you get with a radial gradient. I also changed my stroke to a light red because I'm going to start my rainbow with red. If it doesn't, you can go to Path > Object to Path, and click the Nodes tool, and push the nodes around until you get something closer. When you choose your spiral tool, set the Turns to 0.50, the Divergence to 0.000, and leave the inner radius at 0. Using the Spirals tool, draw a semicircle. One way to do it is to create a rainbow color gradient and fill a semicircle with it. I saw this the other day and thought I'd share.

Inkscape gradient one color to another