Hi Judy!
You have discovered the shakier decisions in Mavericks, the elimination of the Message > Text Encoding setting in Mail. The only workaround I can think of is to temporarily set the OS to Simplified Chinese, and compose the message in the localized Simplified-Chinese Mail. Steps:
[1] Close all applications, or at least Mail.
[2] Open System Preferences > Language & Region and set the primary language to Simplified Chinese.
[3] Open Mail, compose message using Simplified Chinese, send.
[4] Close Mail, and reset the primary language to English.
I tested this with a short message containing some Simplified-only glyphs (e.g., 万), just to be sure to force the message to GB, but I doubt that is really necessary. Anyhow, it worked and the message went out as "GB-2312"...
I don't know if it is necessary to actually compose the message while Mail is localized for Simplified Chinese -- it might be enough to switch to SC when you send -- but I didn't experiment with that.
Eric