Gradients in the style of LSD afterimages were a core part of SC2 and early SC3. Sadly, gradient functionality mostly went away with the port to QT. :(((((
Unfortunately yes. The classes Gradient and HiliteGradient are still documented, but without functionality. For good reason it was not highest priority in the days of the change to Qt, but I always hoped it would come back. GUIs with gradients are much more fun to work with.