The sound of this word can take you towards its sense: the gibbering, gaping gob – like one agog, gaga, gobsmacked – and the siffling crack of smacked like the event that produces the state (and then there is the stunned state that can be produced by the drug sometimes called smack).
Such a good sound for a slap, smacked: the s like the hand whipping through the air, the m giving a sense of the might behind the blow, the aggressive, sharp low front vowel, and then not just the crack with the [k] but the echo with the final [t].
Actually, smack was a word for lip-smacking first: you know, when you make a sharp sound with the sudden parting of the lips, as – evidently more in earlier centuries than now – a sign of gustatory eagerness, such that cognates of this onomatopoeic word in other Germanic languages have to do with flavour and relish. So it’s perhaps ironic that it travelled away from the mouth to the “slap” sense only to be brought back to it with this word, adding gob to the adjectival past participle smacked to signify the point of impact.
Gob is of course another buccal word, suspected to come from Gaelic for “beak”; there is another gob that refers to mouthfuls, lumps, masses, and comes from a French word for the same. So gobstopper refers to a ball of candy (which one might call a gob) that stops up your gob – your mouth – but also stops gobs from coming out of it. (And, though you might drool, you can say “gob” with a gobstopper in – it does not require the blade of the tongue.)
And gobsmacked? Just a striking metaphor, of course. If it seems like a grand old British word, it might be, but there are currently no citations for it before the 1980s, so it might sooner be a grand rather new British word. One way or the other, you’re most likely to find it in predicate position: sooner “I was gobsmacked” than “I saw three gobsmacked Englishmen.”