At the moment, SlickEdit's auto encoding detection doesn't detect any code pages. SlickEdit has a lot of detection for unicode encodings (Utf-8, Utf-16, UTf-32). However, on Windows, if you set the default code page to Chinese Simplified GB2312, then SlickEdit would correctly display this file. Also, you could create a new text file, enter characters from the Chinese Simplified GB2312 code page and save it and the resulting file would be saved in the Chinese Simplified GB2312 code page.
At this point, it's best to use a Unicode encoding to avoid issues with new files you create. Interestingly, the Windows default code page can be configured to be Utf-8 but I think it's still considered experimental (SlickEdit supports it).