It's impossible to authenticate in Gnome 3's lock screen, at least on Fedora. Reported upstream: https://bugzilla.redhat.com/show_bug.cgi?id=960149
From the crashes, it looks like it is blindly assuming it was started from GDM and has delegated responsibility for authentication to it.
Upstream claims to have fixed this now: https://bugzilla.gnome.org/show_bug.cgi?id=726283 Will have to retest once that version of gdm has trickled down to the distributions.
This fixed version of gdm now active in RHEL 7 etc.
The version of Gnome that includes this fix is 3.14.