Here's why:
* It names a thing: A gala is a formal event, usually a celebration.
* It can be the subject of a sentence: "The gala was a huge success."
* It can be the object of a verb: "We attended the gala."
While you might hear phrases like "a gala occasion," "gala attire," or "gala performance," "gala" in those cases is acting as an adjective, describing the occasion, attire, or performance.