It's Brit slang. "Gob" basucally refers to the mouth.
As in: "Shut yer gob!"
Being "gobsmacked" refers to someone who is so surprised and shocked by something, that it is as if they were hit in the mouth, and unable to speak from the shock of it. (whatever caused the stunned amazement.)
Or, *actually* hit in the mouth...
:o)
Love it! Are you British?
My mom used to make these really good chocolate-cake like cookies filled with vanilla frosting. She called them "gobs." They were small and round. When I first read the word all I could think of was someone getting hit by a bunch of "gob" cookies! ;D