is there a way to close the port X11 uses
 

when i scan my computer with nmap show me that the only port whic is open is X11 is there a way to close this port i use x11 just for the screen on the computer.

but i am not starting x11 from command prompt my system directly goes to gdm is there a way to do it for gdm

Did you read this part of that linuxplanet page?

"You may want to put this in a system startup file like /usr/X11R6/lib/X11/xinit/xserverrc or /etc/X11/xinit/xserverrc "

you can make a very simple Iptable script to block that port to everyone except your local machine.
Iptable is the "firewalling" utility for linux.

assuming you use GDM:
Edit file: /etc/X11/gdm/gdm.conf
Make following changes (add the text in red):
[daemon]
.....
StandardXServer=/usr/X11R6/bin/X -nolisten tcp

[server-Standard]
.......
command=/usr/X11R6/bin/X -nolisten tcp