That's a solution to a problem that I don't know exists.
Album gain = preserve the integrity of an album; makes sense when listening to albums.
Track gain: make everything sound as loud; makes sense when shuffling (or picking single tracks from various albums).
What other playback modes are there that would benefit from your solution?
Not agreed. The goal is not to have everything as loud as possible but to have everything appearing as equal loud, and that's the crux.
But that's exactly what track gain does.
I think it's a good idea, thought I think there are too many choices. 25%, 50% and 75% should be adequate, 20% steps will definitely be adequate.
It claims that it does, but it doesn't in practice.
I think a more appropriate GUI control would have been a slider. Having a drop down box was the easiest way to have an implementation without modifying the existing GUI (the drop down box was already there).
Can you demonstrate, even empirically, how it doesn't?
but sometimes I think maybe the differences between quiet tracks and loud tracks in the context of shuffling may be a bit too large in some cases when using straight album gain.
it's obviously useful to you. That should be enough for you to keep and implement that feature wherever you can (unless you don't use your own products!).