I made some progress on at least finding the GUnixMountEntry for a path. Still, there is no way to find GMount out of GUnixMountEntry (GMount is the replacement of GnomeVFSVolume according to https://developer.gnome.org/gio/stable/ch33.html)

What I got so far is that GMount is "user visible mount" - whatever it is. IOW - this is what is seen in "places" in nautilus, IIUC.