Thanks for the quick help and great links. I used the configuration explained in the first link:
and I was able to get a linux PC to authenticate using GDM. I will try KDM later.
I had some little problems as follows:
1- I have to put dhcdbd and nm-applet off and use normal dhcp setup instead.
2- When I disconnect the lan connection,the system takes long time to show GDM logon screen, around 10-15 minutes. This does not happen with XP.
3- The sound is deactivated for the remote users.