'to be amazed"," astounded", "completely taken aback". "to be taken off guard" "surprised to a great degree"

that general sort of feeling, but with the added meaning of also feeling like being "punched in the face"

 "gob" = slang for mouth, "smacked" = hit over the face with the palm of someones hand" .

GOB+SMACK(ed)

google gobsmack

https://en.oxforddictionaries.com/definition/gobsmack