Created attachment 206948 [details]
the test case

The test file, gbk.txt, is from an attachment of the following (old) bug report.
https://bugzilla.redhat.com/show_bug.cgi?id=576047

My environment

$ gedit --version
gedit - Version 3.2.6

$ gsettings get org.gnome.gedit.preferences.encodings auto-detected
['UTF-8', 'CURRENT', 'ISO-8859-15', 'UTF-16']

$ locale
LANG=en_US.UTF-8
LC_CTYPE="en_US.UTF-8"
LC_NUMERIC="en_US.UTF-8"
LC_TIME="en_US.UTF-8"
LC_COLLATE="en_US.UTF-8"
LC_MONETARY="en_US.UTF-8"
LC_MESSAGES="en_US.UTF-8"
LC_PAPER="en_US.UTF-8"
LC_NAME="en_US.UTF-8"
LC_ADDRESS="en_US.UTF-8"
LC_TELEPHONE="en_US.UTF-8"
LC_MEASUREMENT="en_US.UTF-8"
LC_IDENTIFICATION="en_US.UTF-8"
LC_ALL=

$ cat /etc/redhat-release
Fedora release 16 (Verne)

My test procedure

LANG=zh_CN.gbk gedit gbk.txt #FAIL
LANG=zh_CN.gb18030 gedit gbk.txt #FAIL
gedit --encoding gbk gbk.txt #PASS, but no guess here

I also write code to do essentially same testing inside document-output-stream.c. It is a patch to git version I just cloned. It is the first time I submit a patch, please forgive me in case there are mistakes.