Gobsmacked - is an informal word meaning 'speechless'.

astonished, very surprised, astounded

"Gobsmack" remains undefined.

"Gobsmack" is the incorrect form of the word. The correct use is "gobsmacked".

Example:

"I was utterly gobsmacked when I returned home to find the kitten had wrecked the place".

Definition and more examples in the related link below.

it is an expression meaning that a person is amazed or astounded by an event (flabbergasted) it derives from the Irish language.

Gob Smacking, or "Gobsmacked" is slang expression used in the UK to indicate surprised, astonished. "Gob" is slang for one's mouth. When a person is amazed or astonished, he might put the open palm of his hand over his open (in astonishment) mouth, simulating a "smack," hence, Gobsmacking. In a sentence: "When I found out I won the lottery, I was gobsmacked."