ubuntu GB18080 to utf-8

I copied some txt files from windows to linux. Then opened it. I found that vim cannot render the text correctly. I know this was caused by GBK encoding. I tried to open it with gedit, it's still messy code. Then I found that, if you go to the "open" of gedit, you can specify the encoding manually. But there were only two encoding, I add GB18030, and open the file, then everything was fine.

In text mode, we can use this command to do the conversion as well.
iconv -f GB18030 -t utf-8 4.txt > 4_U8.txt