This Girls of Gossip Girl photo contains
robe de cocktail and gaine.
There might also be
bustier, robe de dîner, formel, robe de soirée, formelle, robe costume, robe complète, frac, manteau de queue, queues, cravate blanche, cravate blanche et la queue, robe longue, tailcoat, tail coat, tails, cravate blanche et queues, habit, robe pleine, and queue de pie.