Thanks, Chris, this was a good tip.
I've made some progress.

With the following arguments (no need for a gsettings-desktop-schemas
input anymore):

--8<---------------cut here---------------start------------->8---
(arguments
    `(#:glib-or-gtk? #t
      #:phases
      (modify-phases %standard-phases
        (add-after 'install 'wrap
          (lambda* (#:key outputs #:allow-other-keys)
            (let ((out (assoc-ref outputs "out")))
              (wrap-program (string-append out "/bin/gnome-music")
                `("PYTHONPATH" ":" prefix (,(getenv "PYTHONPATH")
                                           ,(string-append out "/lib/python"
                                                           ,(version-major+minor
                                                             (package-version python))
                                                           "/site-packages")))
                `("GI_TYPELIB_PATH" ":" prefix (,(getenv "GI_TYPELIB_PATH")))
                `("GRL_PLUGIN_PATH" ":" prefix (,(getenv "GRL_PLUGIN_PATH")))))
            #t)))))
--8<---------------cut here---------------end--------------->8---

gnome-music starts.

Sadly it displays

"GNOME Music could not connect to Tracker"

Tracker is part of the inputs but I guess that's not enough.
I tried installing gnome-music to my default profile, then run it from a
GNOME session, to no avail.

-- 
Pierre Neidhardt
https://ambrevar.xyz/