Install the following package
$apt-get install xnest
and give the command
$gdmXnestchooser
It will show screen like in the figure
Normally you will see the server in the window. Else enter the ip of the server
Then double click on the server’s button
You will see a logon window like this
Then login and you can see the Xmanager type session here
If u want a full screen logon in gnome goto applications–>system–>newlogin
and in the actions choose run xdmcp server. Then you will see the same screen