This is the 4th time Georgina (Michelle Trachtenberg) has returned for a season finale, so she has returned for every season finale so far.
David O. Russell (who appears as himself in this episode) has worked as a writer/director/producer on thee movies that starred Jennifer Lawrence: Silver Linings Playbook (2012), American Hustle (2013) and Joy (2015). Jennifer had auditioned to play Serena before Blake Lively had been cast.
It's been shown and mentioned many times that Blair is a fan of Classic American movies. Ethan Peck (David O. Russell's Assistant) is the grandson of actor Gregory Peck, who appeared in one of Blair's favorite movies, Roman Holiday (1953), playing Joe Bradley.
Ethan Peck (David O. Russell's Assistant) is the grandson of actor Gregory Peck, who appeared with Sam Robards's (Howie 'The Captain' Archibald but doesn't appear in this episode) mother, Lauren Bacall in the movie, Designing Woman (1957). They played Mike Hagen and Marilla Brown Hagen respectively.
After Season 4, both Jessica Szohr and Taylor Momsen were initially downgraded from the main cast to recurring status, and then written off the show entirely except for the cameos they each made in the series finale at the end of Season 6. Szohr was written out because the producers felt her character had "run its course", while Momsen's departure was due to a combination of her wanting to stop acting and focus on her musical career & the fact that the cast and crew were tired of dealing with her unprofessional attitude.
Cecily von Ziegesar: author of the Gossip Girl book series, as the lady Headmistress Queller introduces to Serena at the party.