I asked if Music supported ReplayGain on IRC, and was told that it didn't, but that I should file a bug if I wanted to see it.

Aside from simply begging, here's an idea on how to intelligently support it:

Use album ReplayGain levels when playing selections directly from the music library (falling back on "track" ReplayGain tag is that's all that is available).

Use track ReplayGain levels when playing from a playlist, and the upcoming track is not what would immediately follow the current track on its album?

I'm not exactly sure how others use ReplayGain, but I think that's probably a "good enough" start.

Thanks!