I didn't see any usage instructions anywhere; not even on github. So, all can presume is the Gradient Selection effect should be visible during text selection. I dont see any difference whatsoever. I even disabled the productivity pack extension; however, that didnt make a difference when selecting.
The selection is nice, but I've problem with it. I use "Productivity Power Tools" extension and in my environment the "Gradient Selection" causes Ctrl+Click (go to definition) feature is not functional. When I disable your extension, the feature is functional again.
I haven't looked into it, I haven't spent much time on the extensions in the last year or so.
If anyone else wants to mess around with it, the source for the extensions are:
Gradient Selection: https://github.com/NoahRic/GradientSelection
It uses the system's defined highlight color, which is why changing the selection background color in VS has no effect. I did it mostly that way from laziness, sorry :(
If you want, you can download the source (http://github.com/NoahRic/GradientSelection) and modify it with a different selection color. I'll see about updating the extension, though I can't make any guarantees on when I'll have time to do it :-/