If you received a gift certificate for a free meal or for a free spa treatment, call the manager before you go and ask if the gratuity is included. If it's not, ask for the estimated value of the gift certificate, and then tip in cash based upon the price you have been given.
You should tip him or her the same as you would tip any member of the staff. Customers are expected to tip as they would at any other place. In the "old days" one didn't tip the owner, but times have changed and it is now proper to do so.