Aegisub is not the solution to everything. In case of gradience/clip creation, there's no real automated way of doing it. You'll need to learn the \clip override function for ASS, and enter it in the line.
This post contains a sample code including clip.
And when jfs said "it's only two halves in slightly different colour," he's saying that the line can be made with two lines -- first line that's \clipped to show the top half of the line with darker color, and the second line that's \clipped to show the bottom half.
For more information about clip, read the Aegisub help file that comes with version 1 -- Help -> Contents -> Typesetting -> Ass Overide Tags, and find the section about \clip. It has pretty good explanation with a sample screen.