What does gala mean as a prefix?

There isn't a prefix called "gala." It's likely you're thinking of the word "gala" itself, which is a noun meaning:

* A large, formal social gathering, especially one held to celebrate a special occasion.

For example, you might attend a "gala dinner" or a "gala performance."

There are no prefixes that are derived from or use the word "gala." If you can provide more context about where you saw or heard "gala" as a prefix, I might be able to give you a more helpful answer!