“gnome-fallback” is no longer a valid session name for org.gnome.desktop.session/session-name. It was removed in 2013:
commit bef001f9b0fe41ef731810e6520dcead5bd55334 Author: Matthias Clasen mclasen@redhat.com Date: Fri Jan 25 16:49:59 2013 -0500
Remove the fallback session
We no longer support fallback mode in 3.8, so we don't need this
session definition anymore. We are keeping the runnable helper,
since it sets some X properties that we need.
https://bugzilla.gnome.org/show_bug.cgi?id=687586fixes #33 (closed)