sudo gedit doesn't work

Help! I am trying to edit a postinfo.xsl in my root file dir, and with the command:

frank@linux-avnb:~> sudo gedit

(gedit:14150): Gtk-WARNING **: cannot open display:

I get the following. what am i doing wrong?

Thanks in advance, Frank

Hit ALT+F2 to raise the run command, and enter

gnomesu gedit

personally i dislike “sudo” and its variants
so if you DO HAVE a /root folder I would use " su -"

su -
---- your root password when asked--

i also DO use a different password for root and for a normal user
suse defaults to the same password for both , i reset it on my system

IMO ‘gnomesu gedit’ is better than ‘su -’ +password + ‘gedit’