one orf the'phrase origins' sites has this:

"Gob" is a very old (about 400 years old, actually) English dialect word meaning "mouth," probably taken from Gaelic or Scots, and related to "gab," also meaning "mouth" or, more commonly, "speech." To be "gobsmacked" is to be astonished or flabbergasted, as stunned as if you had been suddenly "smacked" (struck) in the mouth.
    Curiously, "gobsmacked" has only been found in print as far back as the 1980s, but it's reasonable to assume that the term has been around for much longer.