I had unused credit (gift certificate) at a spa that closed - bankrupt. How can I get that $$ back?

No. You're holding an obligation of the company. The company filed bankruptcy likely because their debts overwhelmed them. The goal of a bankruptcy by any filer is to have as many of their debts discharged as possible. More likely than not you are out of luck.

Three potential exceptions: (1) Sometimes, a company like a spa or a gym going out of business will sell their memberships or outstanding gift certificates to another entity, it gives the seller some fast cash and it gives the buyer a list of potential new customers. Watch the local newspapers. (2) Sometimes in a Chapter 11 a court could allow the membership list to be sold, watch the notices in the newspapers, (3) Often when a gym or spa goes out of business in a town where there are other businesses like that, the other ones will have an advertising campaign to get the defunct company's customers by honoring the memberships or gift certificates. Keep your eyes opes for such a deal.