I finally found a way of getting around the feared

  (gnumeric:1974): GLib-GIO-ERROR **: Settings schema 'org.gtk.Settings.FileChooser' is not installed

by setting XDG_DATA_DIRS before running a GTK tool (gnumeric, gnucash,
some browsers):

  env XDG_DATA_DIRS=/usr/local/share:/usr/share ~/.guix-profile/bin/gnumeric

This is on a Debian base distribution. Not sure what package is
missing on Guix, but there appears to be a missing dependency.