Output Package gdm Error | --distribution = wheezy

[teseo@broletto.org]

Hello all,

thi is my output error when change the only option --distribution from
squeeze to wheezy. here are the packages included in wheezy:

debian-installer-launcher
build-essential
terminator
chromium-browser
mutt
irssi
rtorrent
remmina
sylpheed
anjuta
liferea
evince
gparted
vim
git

***
Package gdm is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However the following packages replace it:
  indicator-applet-session

Package live-initramfs is not available, but is referred to by another
package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'live-initramfs' has no installation candidate
E: Package 'gdm' has no installation candidate
P: Begin unmounting filesystems...
***

No problem if build the script with squeeze option.

many thanks at all

marco