Give an example of people who benefit from gain during an unanticipated deflation.

Any creditor who lends money to borrowers at a fixed interest rate suffers from loss during an unanticipated deflation.

e.g. banks who offer mortgage loans to borrowers

(Note that investors who hold inflation-linked bonds instead of cash do not suffer from loss this time because inflation-linked bonds guarantee a minimum percentage of interest rate even when there is no inflation.)