Leighton Meester (Blair Waldorf) and Melissa Fumero (Zoe) have both worked with Andy Samberg in separate projects: Leighton worked with Andy in, That's My Boy (2012) playing Jamie and Todd respectively and Melissa worked with Andy in, Brooklyn Nine-Nine (2013) playing Amy Santiago and Jake Peralta respectively.
Rachel Brosnahan (Girl) has been married to actor Jason Ralph since 2016. In the previous episodes, There Might be Blood (2008) and The Goodbye Gossip Girl (2009), Stella Maeve makes appearances as the character of Emma Boardman. Jason and Stella worked together in the TV show, The Magicians (2015) playing best friends, Quentin Coldwater and Julia Wicker respectively.
Lily and Rufus briefly make mention of writer/director Nancy Meyers, whilst in their apartment. Melissa Fumero (Zoe) ends up working on the TV show, Brooklyn Nine-Nine (2013), playing Amy Santiago and one of Amy's colleagues, Rosa Diaz (played by Stephanie Beatriz), is a big fan of Nancy Meyers. It's a recurring gag of the show.
Of interest to Taylor Momsen fans, arguably the best Jenny scene got cut. She admires Chuck for being 'pretty smart' and he arranges a meeting with her. He says that he noticed she's grown up, even if Nate doesn't. He then looks at her like quite the wolf, and she seems all too willing to be led by the wolf up the garden path.
Rachel Brosnahan (Girl) ends up working with an actor from the rebooted show, Gossip Girl (2021), in the TV show, The Marvelous Mrs. Maisel (2017). The actor in question is, Luke Kirby, who played Davis Calloway in the reboot. They played Midge Maisel and Lenny Bruce respectively, with their characters being close friends and one-time lovers.