Adj. Flabbergasted; speechless. Note: “gobsmacked” is a relatively new U.K. slang term; its first Oxford English Dictionary citation is from 1985. It derives from a word for “mouth” used in northern England, “gob.” “When I saw Minority Report, I was gobsmacked by it…” (Roger Ebert, Slate, July 3, 2003)/”[In the film, How Do You Know, Paul] Rudd registers a little soft, and not nearly stunned or rancorous enough, for a guy who’s been gobsmacked by fate.” (Time Magazine, December 17, 2010) Google Ngram.
notoneoffbritishisms.com 
This one has become common in the US as a result of Susan Boyle.
“Gobsmacked” is without a doubt one of the ugliest words in the English language. Sounds just horrible. And from an American mouth, it always sounds forced, unnatural. In American English it’s a vogue word, and I predict it won’t last.
The Paul Rudd use is actually wrong. You aren’t be gobsmacked BY something. The sense is of being rendered speechless, coming up against a brick wall of disbelief. “I’m gobsmacked by your attitude” invalidates itself. “I’m gobsmacked” demands a full stop. (period)
I disagree with this assessment.
In the Paul Rudd quote, he’s talking about something in the past tense. If you’re relating a story to someone, you’d obviously tell them why you were gobsmacked, e.g. “I was just gobsmacked by his attitude, you know? I had to walk away.”
Now, your example is in present tense, and at least as I’ve interpreted it you’re implying its usage as an exclamatory interjection of sorts in an exchange between two or more parties, akin to, “Well, I never!” But spoken English has tons of idiosyncrasies, and it wouldn’t be extraordinary to hear something like this in an argument: “I’m gobsmacked you just said that to me, you prat!” 😉
Regardless of any particular colloquial or personal preference to its usage, “gobsmacked” is an adjective that can be used like any other in its proper context. I’m not sure why you would think otherwise.
Gob is the Irish for mouth so I’d guess it mades its way from Irish-English to British-English by way of “Shut yer Gob” and similar phrases. I think gobsmacked is a relatively recent evolution though. Its the type of phrase used by tabloid news hacks to describe someone as being shocked.
“Gob” the common word for “mouth” in northern english and scottish dialects too, i.e. not exclusively Irish. Compare also “gobby” (mouthy) i.e. shoot your mouth off, and “gobshite”.
This might have entered by way of HP?
Thumbs up. I actually used that word in my novel…liked it.
Gobsmacked was certainly in use in my 1950’s childhood in Hull (oop North).
Gob is certainly Irish, and like a lot of such words was probably adopted originally by Liverpudlians.
I’m commenting here loooong after this item was posted but was put in mind of it by the new “bag of sweets” item. “Gobsmacked,” like “ghastly,” can be uttered without embarrassment by only Brits. Neither sounds at all right (i.e., correct, mellifluous) coming out of a (North) American mouth.
‘Gob’ is also used in Australia.
In this episode of Mythbusters, Adam Savage (who one suspects to be a closet Anglophile) says he is “gobsmacked” at the damage wrought by the Breaking-Bad style automachinegun
http://www.discovery.com/tv-shows/mythbusters/videos/machine-gun-booby-trap/