由于我不能要求别人保存txt文件时必须用utf-8，那我只能自己找解决办法：
打开终端输入：
gsettings set org.gnome.gedit.preferences.encodings auto-detected "['GB18030', 'GB2312', 'GBK', 'UTF-8', 'BIG5', 'CURRENT', 'UTF-16']"
gsettings set org.gnome.gedit.preferences.encodings shown-in-menu "['GB18030', 'GB2312', 'GBK', 'UTF-8', 'BIG5', 'CURRENT', 'UTF-16']"
解决乱码问题.