I don't have a file encoded in GBK to test the solutions, but do try them.

Option 1: Run gedit and then open the file:

• run gedit




• click open




• select in the encoding combo "add" and add GB18030 (or GBK)




• select that encoding in the combo




• select the file

Option 2:

• install and run gconf-editor




• Find /apps/gedit-2/preferences/encoding key "auto_detected" and set it to:

  
  
  

  [CURRENT,GB18030,GBK,GB2312,UTF-8,UTF-16]
  
  

Remember to backup the previous value of auto_detected!!

Option 3:
Using dconf (the "newer" gconf, used in ubuntu)

dconf read /org/gnome/gedit/preferences/encodings/auto-detected

Save the result (backup)!

dconf write /org/gnome/gedit/preferences/encodings/auto-detected "['CURRENT', 'GBK', 'GB18030', 'GB2312', 'UTF-8', 'UTF-16']"

Sources:

• https://bugs.launchpad.net/ubuntu/+source/gedit/+bug/819714


• https://bugzilla.redhat.com/show_bug.cgi?id=576047#c2


• http://ubuntuforums.org/showthread.php?t=1456308


• http://forum.ubuntu-gr.org/viewtopic.php?p=221205&http#p221205

To reset it to default:
a) use the option you backed up from dconf read command

or b) try:

dconf reset /org/gnome/gedit/preferences/encodings/auto-detected