Blake Lively (Serena van der Woodsen) and Chace Crawford (Nate Archibald) both end up working with Anna Kendrick in separate projects: Blake worked with Anna in the movie, A Simple Favor (2018) playing Emily Nelson and Stephanie Smothers respectively and Chase worked with Anna in the movie, What to Expect When You're Expecting (2012) playing Marco and Rosie respectively.
Connor Paolo (Eric van der Woodsen) ends up working on the TV show Revenge (2011), which also stars Emily Alyn Lind, playing Declan Porter and Young Amanda Clarke respectively. Emily appears in the reboot, Gossip Girl (2021) playing Audrey Hope.
Of the original main cast, two of them are left handed: Penn Badgley (Dan Humphrey) and Ed Westwick (Chuck Bass).
Zuzanna Szadkowski (Dorota Kishlovsky) ends up working with Leighton Meester's (Blair Waldorf) real-life husband, Adam Brody in the movie, Growing Up and Other Lies (2014). They played CeCe and Rocks respectively.