This almost sounds more like a stomach thing (H. pylori, etc.) than the gallbladder.
"Diseased gallbladder" seems like an awfully vague diagnosis. What is the name of this disease? How can he tell that you have it? Is there no other way to treat it? (Once you know the name of this disease you can research it on the Web.)
Perhaps you might want to consider getting a second opinion before taking such a big step...