Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 213

You will now be able to hold the mouse button and move the center
triangle on the slider. When you're satisfied, enter a name for the new custom
gradient in the Gradient Editor window and
save it by clicking the Save button (bottom
left in Figure 3.36).
A final note about the color picker: You
can also add Transparency to a gradient
blend to create interesting effects. To
decrease the opacity of a gradient, change
the value (A) in the color picker window.
The default setting for opacity is 100%.
Next, you have to apply the gradient
blend to the intended layer of your image.
Make sure that you've selected the
desired gradient blend in the Gradients
window. Then select the Gradient tool from
the Tools palette and double-click on it to
reveal the tool options.

Figure 3.38
Color sliders for gradient blends with an option to set the opacity (A)

Figure 3.39
The Toolbox and Gradients window with the selected gradient blend and the Blend tool options