If you knew which file(s) were missing and/or corrupt, you could certainly boot from the live-usb, mount your hard drive installation at some convenient location, and copy the required files from the live system to the hard drive. Not something I would normally recommend, but certainly something I would do myself.
Not sure where or how gdm write its config info, but on my systems (Fedora 16 and Rawhide running LXDE), I see:
Code:
~$ sudo cat lxdm.conf
[base]
# autologin=dgod
session=/usr/bin/startlxde
# numlock=0
greeter=/usr/libexec/lxdm-greeter-gtk
[server]
arg=/usr/bin/X -background vt1
[display]
gtk_theme=Clearlooks
bg=/usr/share/backgrounds/default.png
bottom_pane=1
lang=1
theme=Industrial
[input]
[userlist]
disable=0
white=
black=
/etc/lxdm$ sudo cat /etc/lxdm/lxdm.conf
[base]
# autologin=dgod
session=/usr/bin/startlxde
# numlock=0
greeter=/usr/libexec/lxdm-greeter-gtk
[server]
arg=/usr/bin/X -background vt1
[display]
gtk_theme=Clearlooks
bg=/usr/share/backgrounds/default.png
bottom_pane=1
lang=1
theme=Industrial
[input]
[userlist]
disable=0
white=
black=
