It should be possible to select the Music directory.

Gnome-music defaults to ~/Music, and doesn't appear to let you configure that.

My music isn't in ~/Music, because ~ is too small.  There's a second disk.  ~/Music is a symbolic link there.

Tracker doesn't follow symbolic links, so I'm indexing the other directory in tracker-preferences, and 'tracker search --music' finds these files, albums and artists.

gnome-music doesn't find those files.  There's a helpful message:
WARNING ~/Music is a symlink, this folder will be omitted

query.py has "Glib.UserDirectory.DIRECTORY_MUSIC", but I can't find a setting for that in gnome-settings nor gnome-tweak-tool nor gnome-music itself.

I've been able to set it using xdg-user-dirs-update, but that doesn't seem discoverable.