Drain orange, measuring syrup. Add water to syrup to make 1 cup. Dissolve gelatin in boiling water. Add measured liquid. Add sherbet by spoonfuls, stirring until melted. Chill until thickened.
Fold in oranges, reserving a few for garnish. Pour into a serving bowl or individual serving dishes. Chill until set-at least 1 hour. Garnish with reserved mandarin orange sections and mint leaves, if desired.