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Gala means a festive occasion, especially a lavish social event or entertainment. Because of the intense curiosity from people near and far over this trial there is a sense of this "gala" event. The irony is it is over something very serious and sombre.
To keep it simple, Scout refers to the trial as a "gala event" because many people have gathered to watch it as though it is some sort of premiere. They treat the trial as a lavish event rather than a court hearing.