I think that is the expression Brit's use when they are very impressed by something.
Well, I am gob smacked by the performance of Susan Boyle, a middle aged Scottish villager who appeared on Britain's Got Talent and blew away the competition.
Here is a link to the You Tube video of her performance.
You must listen to her... her voice literally brought tears to my eyes and a grin to my face.
Thanks to Janetta for the link!