Steps to reproduce:
1. install gdm
2. make sure there are other xsessions but gnome.desktop present in /usr/share/xsessions and working (KDE, Xfce, LXDE or alike)
3. remove gnome.desktop. In Fedora thins can be done by uninstalling gnome-session-xsession


Stack trace:
gdm hangs after selecting the name and before displaying the password prompt. Session selection dropdown is greyed out. Cancel button does not work because gdm-simple-slave has crashed.

/var/log/gdm/:0-greeter.log:
gdm-simple-greeter[8909]: WARNING: Unable to start greeter session: Failed to connect to socket /tmp/gdm-greeter-EFMxqeGE: Verbindungsaufbau abgelehnt
(means "Connection denied")

/var/log/gdm/:0-slave.log:
*** glibc detected *** /usr/libexec/gdm-simple-slave: double free or corruption (fasttop): 0x08725140 ***
======= Backtrace: =========
/lib/libc.so.6[0x1843a4]
/lib/libc.so.6(cfree+0x96)[0x186356]
/lib/libglib-2.0.so.0(g_free+0x36)[0x8e08e6]
/usr/libexec/gdm-simple-slave[0x8056520]
/usr/libexec/gdm-simple-slave[0x8056773]
/usr/libexec/gdm-simple-slave[0x80574b5]
/usr/libexec/gdm-simple-slave[0x80576c6]
/lib/libgobject-2.0.so.0(g_cclosure_marshal_VOID__STRING+0x8c)[0xd4614c]
/lib/libgobject-2.0.so.0(g_closure_invoke+0x1ab)[0xd391fb]
/lib/libgobject-2.0.so.0[0xd4f655]
/lib/libgobject-2.0.so.0(g_signal_emit_valist+0x7dc)[0xd50d6c]
/lib/libgobject-2.0.so.0(g_signal_emit+0x26)[0xd511e6]
/usr/libexec/gdm-simple-slave[0x804e323]
/lib/libdbus-1.so.3[0x70c765]
/lib/libdbus-1.so.3(dbus_connection_dispatch+0x404)[0x6fe1e4]
/usr/lib/libdbus-glib-1.so.2[0xdca08d]
/lib/libglib-2.0.so.0(g_main_context_dispatch+0x1e8)[0x8d8258]
/lib/libglib-2.0.so.0[0x8db903]
/lib/libglib-2.0.so.0(g_main_loop_run+0x1d2)[0x8dbe22]
/usr/libexec/gdm-simple-slave[0x804cb31]
/lib/libc.so.6(__libc_start_main+0xe5)[0x12b6e5]
/usr/libexec/gdm-simple-slave[0x804c751]
======= Memory map: ========
00110000-00114000 r-xp 00000000 fd:00 344301     /lib/libattr.so.1.1.0
00114000-00115000 rw-p 00003000 fd:00 344301     /lib/libattr.so.1.1.0
00115000-00283000 r-xp 00000000 fd:00 344280     /lib/libc-2.9.so
00283000-00285000 r--p 0016e000 fd:00 344280     /lib/libc-2.9.so
00285000-00286000 rw-p 00170000 fd:00 344280     /lib/libc-2.9.so
00286000-00289000 rw-p 00286000 00:00 0 
00289000-0028c000 r-xp 00000000 fd:00 344478     /lib/libdl-2.9.so
0028c000-0028d000 r--p 00002000 fd:00 344478     /lib/libdl-2.9.so
0028d000-0028e000 rw-p 00003000 fd:00 344478     /lib/libdl-2.9.so
0028e000-0028f000 r-xp 00000000 fd:00 935122     /usr/lib/libxcb-xlib.so.0.0.0
0028f000-00290000 rw-p 00000000 fd:00 935122     /usr/lib/libxcb-xlib.so.0.0.0
00290000-0029b000 r-xp 00000000 fd:00 344235     /lib/libnss_files-2.9.so
0029b000-0029c000 r--p 0000a000 fd:00 344235     /lib/libnss_files-2.9.so
0029c000-0029d000 rw-p 0000b000 fd:00 344235     /lib/libnss_files-2.9.so
002fc000-003f9000 r-xp 00000000 fd:00 941350     /usr/lib/libX11.so.6.2.0
003f9000-003fc000 rw-p 000fd000 fd:00 941350     /usr/lib/libX11.so.6.2.0
003fc000-003fd000 rw-p 003fc000 00:00 0 
00674000-0067f000 r-xp 00000000 fd:00 344317     /lib/libpam.so.0.81.13
0067f000-00680000 rw-p 0000a000 fd:00 344317     /lib/libpam.so.0.81.13
0068c000-006a4000 r-xp 00000000 fd:00 344298     /lib/libaudit.so.0.0.0
006a4000-006a5000 r--p 00018000 fd:00 344298     /lib/libaudit.so.0.0.0
006a5000-006a6000 rw-p 00019000 fd:00 344298     /lib/libaudit.so.0.0.0
006e9000-006ed000 r-xp 00000000 fd:00 344602     /lib/libcap.so.2.10
006ed000-006ee000 rw-p 00003000 fd:00 344602     /lib/libcap.so.2.10
006f0000-0072f000 r-xp 00000000 fd:00 346841     /lib/libdbus-1.so.3.4.0
0072f000-00730000 r--p 0003e000 fd:00 346841     /lib/libdbus-1.so.3.4.0
00730000-00731000 rw-p 0003f000 fd:00 346841     /lib/libdbus-1.so.3.4.0
00733000-00749000 r-xp 00000000 fd:00 346842     /lib/libnsl-2.9.so
00749000-0074a000 r--p 00016000 fd:00 346842     /lib/libnsl-2.9.so
0074a000-0074b000 rw-p 00017000 fd:00 346842     /lib/libnsl-2.9.so
0074b000-0074d000 rw-p 0074b000 00:00 0 
00762000-0076a000 r-xp 00000000 fd:00 935431     /usr/lib/libXrender.so.1.3.0
0076a000-0076b000 rw-p 00007000 fd:00 935431     /usr/lib/libXrender.so.1.3.0
0076d000-00771000 r-xp 00000000 fd:00 935552     /usr/lib/libXfixes.so.3.1.0
00771000-00772000 rw-p 00003000 fd:00 935552     /usr/lib/libXfixes.so.3.1.0
00774000-00794000 r-xp 00000000 fd:00 344246     /lib/ld-2.9.so
00795000-00796000 r--p 00020000 fd:00 344246     /lib/ld-2.9.so
00796000-00797000 rw-p 00021000 fd:00 344246     /lib/ld-2.9.so
008a1000-00983000 r-xp 00000000 fd:00 344219     /lib/libglib-2.0.so.0.1800.4
00983000-00984000 rw-p 000e2000 fd:00 344219     /lib/libglib-2.0.so.0.1800.4
009a8000-009a9000 r-xp 009a8000 00:00 0          [vdso]
00b05000-00b20000 r-xp 00000000 fd:00 935132     /usr/lib/libxcb.so.1.0.0
00b20000-00b21000 rw-p 0001b000 fd:00 935132     /usr/lib/libxcb.so.1.0.0
00bcc000-00bd1000 r-xp 00000000 fd:00 434354     /usr/lib/libXdmcp.so.6.0.0
00bd1000-00bd2000 rw-p 00004000 fd:00 434354     /usr/lib/libXdmcp.so.6.0.0
00bda000-00bdc000 r-xp 00000000 fd:00 434353     /usr/lib/libXau.so.6.0.0
00bdc000-00bdd000 rw-p 00001000 fd:00 434353     /usr/lib/libXau.so.6.0.0
00c9c000-00ca5000 r-xp 00000000 fd:00 935681     /usr/lib/libXcursor.so.1.0.2
00ca5000-00ca6000 rw-p 00008000 fd:00 935681     /usr/lib/libXcursor.so.1.0.2
00d30000-00d70000 r-xp 00000000 fd:00 344267     /lib/libgobject-2.0.so.0.1800.4
00d70000-00d71000 rw-p 00040000 fd:00 344267     /lib/libgobject-2.0.so.0.1800.4
00da0000-00dbb000 r-xp 00000000 fd:00 344180     /lib/libselinux.so.1
00dbb000-00dbc000 r--p 0001a000 fd:00 344180     /lib/libselinux.so.1
00dbc000-00dbd000 rw-p 0001b000 fd:00 344180     /lib/libselinux.so.1
00dc3000-00de0000 r-xp 00000000 fd:00 940918     /usr/lib/libdbus-glib-1.so.2.1.0
00de0000-00de1000 rw-p 0001d000 fd:00 940918     /usr/lib/libdbus-glib-1.so.2.1.0
03e14000-03e16000 r-xp 00000000 fd:00 344559     /lib/libutil-2.9.so
03e16000-03e17000 r--p 00001000 fd:00 344559     /lib/libutil-2.9.so
03e17000-03e18000 rw-p 00002000 fd:00 344559     /lib/libutil-2.9.so
04f9a000-04fa7000 r-xp 00000000 fd:00 346854     /lib/libgcc_s-4.3.2-20081105.so.1
04fa7000-04fa8000 rw-p 0000c000 fd:00 346854     /lib/libgcc_s-4.3.2-20081105.so.1
08048000-0806f000 r-xp 00000000 fd:00 941587     /usr/libexec/gdm-simple-slave
0806f000-08072000 rw-p 00026000 fd:00 941587     /usr/libexec/gdm-simple-slave
086fa000-08736000 rw-p 086fa000 00:00 0          [heap]
b7d00000-b7d21000 rw-p b7d00000 00:00 0 
b7d21000-b7e00000 ---p b7d21000 00:00 0 
b7e29000-b7e48000 r--p 00000000 fd:00 935048     /usr/share/locale/de/LC_MESSAGES/libc.mo
b7e48000-b7e52000 r--p 00000000 fd:00 936530     /usr/share/locale/de/LC_MESSAGES/glib20.mo
b7e52000-b8052000 r--p 00000000 fd:00 458793     /usr/lib/locale/locale-archive
b8052000-b8057000 rw-p b8052000 00:00 0 
b805d000-b8064000 r--s 00000000 fd:00 975509     /usr/lib/gconv/gconv-modules.cache
b8064000-b8069000 r--p 00000000 fd:00 938131     /usr/share/locale/de/LC_MESSAGES/gdm.mo
bfb53000-bfb68000 rw-p bffeb000 00:00 0          [stack]

Other information:
Why is gdm looking for gnome.desktop? The user I tried to login has never used Gnome but Xfce.

Initially reported at https://bugzilla.redhat.com/show_bug.cgi?id=483497