Everything started when I edit the file /etc/pam.d/gdm adding the line
@include common-pamkeyring
to unlock the keyring

When I rebooted I could not login with my usual password.
So I tried to boot in recovery mode using bash shell as root to edit the file again deleting that line, but nothing, the problem was still there.
So, using live cd, I used the shell as root to browse into my home directory and delete the file:
.gnome2/keyrings/default.keyring
this file was not there, there was only the file
.gnome2/keyrings/login.keyring
and I deleted it.

Now the problem is still there and here's the strange fact: when I type atlogin my usual user/pass combo, there's a little popup window which claims "Authentication failed", while if I type my user with another wrong password, the same login window shows, as it should be, that user or pass are wrong, etc. etc.

Is there a "painless" way to solve the problem or I really have to reinstall from scratch?

Can you log into one of the virtual terminals? Once you are in the terminal, you can try to fix the situation or start X windows:
xinit -- :0.0
will start the xserver and run xterm by default. From the xterm, you could start a window session.

You indicated adding a line to gdm, the that line implies that you have modified your PAM configuration. Getting the PAM configurations wrong can lock you out, as you have found. The only file you mentioned changing is a gdm config file. Logging in to a virtual terminal and starting X using xinit, you won't be running gdm.