Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 212

that the left section turns dark gray, while the right half turns light gray. Next,
open the context menu and select the Right Endpoint's Color option. Double-
click the right half of the slider, and then open the context menu and select
the Load Left Color From > Left Neighbor's Right Endpoint option.
Figure 3.37
The context menu in the Gradient Editor