gedit cannot use 'save as and change name' to create a new file.

Does this happen every time you try to use Save As... in gedit, or just some of the time? If the latter, can you provide details about when it happens and when it does not?

Please also open a Terminal window (Ctrl+Alt+T) and run this command:

lsb_release -a; echo; apt-cache policy gedit

The best way is to copy the command from here, paste it in the Terminal, and press enter. That avoids typos.

Then please select all the text in the Terminal (Edit > Select All), copy it to the clipboard (Edit > Copy), and paste it here.

Thank you Eliah. It happens very time.
The text is as follows.

zhangz@zhangz-desktop:~$ lsb_release -a; echo; apt-cache policy gedit
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 10.10
Release: 10.10
Codename: maverick

gedit:
  Installed: 2.30.3-1ubuntu1
  Candidate: 2.30.3-1ubuntu1
  Version table:
 *** 2.30.3-1ubuntu1 0
        500 http://debian.ustc.edu.cn/ubuntu/ maverick/main amd64 Packages
        100 /var/lib/dpkg/status

Thank you Eliah. It happens very time.
The text is as follows.

zhangz@zhangz-desktop:~$ lsb_release -a; echo; apt-cache policy gedit
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 10.10
Release: 10.10
Codename: maverick

gedit:
  Installed: 2.30.3-1ubuntu1
  Candidate: 2.30.3-1ubuntu1
  Version table:
 *** 2.30.3-1ubuntu1 0
        500 http://debian.ustc.edu.cn/ubuntu/ maverick/main amd64 Packages
        100 /var/lib/dpkg/status

how did you invoke gedit?

if you started it from the gnome desktop it will save your file.

if you're starting it from terminal, you'll need to type

gksu gedit ...

it sounds like you invoked it from the command line.

What files are you editing with gedit?

You do NOT need to run gedit with gksu or gksudo unless you want to run it as root (to edit a file you cannot otherwise edit due to permissions, such as a systemwide configuration file). You should NOT ever run gedit with gksu or gksudo unless you know you have to. It is *very bad* to run programs as root without a reason, because you could accidentally change or overwrite any file without meaning to, and badly break your Ubuntu system. You can also render your own files (reversibly) inaccessible to you, by creating or editing them as root.

As a separate matter, you should also not run gedit (ever) directly with sudo (see https://help.ubuntu.com/community/RootSudo). Perhaps that is what Marcus was concerned about. But if you are using gedit to edit your own files, you should not run it with gksu (nor with gksudo).

@marcus
gedit runs fine from the Terminal as a limited user. The gedit desktop shortcut doesn't use gksu or any other privilege elevation mechanism, and users running gedit from the Terminal generally should not do so either.