While HBO Max's "Gossip Girl" reboot has already garnered an impressive following of old and new fans alike, we can't help but reminisce about the show's original run! Join Wonderwall.com as we take a look back at all the stars we forgot appeared on the original "Gossip Girl"… starting with Sebastian Stan, who celebrates his 39th birthday on Aug. 13, 2021. That's right — before joining the Marvel Cinematic Universe as Bucky Barnes a.k.a. the Winter Soldier, Sebastian portrayed Upper East Side's bad boy, Carter Baizen, for the first three seasons. In addition to being a longtime rival of Chuck and Nate's, Carter also engaged in a complicated, on-again off-again romance with Blake Lively's Serena van der Woodsen.
Keep reading to see more stars who've appeared on the hit teen drama…
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Hilary Duff appeared on "Gossip Girl" during the show's third season as Olivia Burke, a movie star who attends NYU and briefly dates Dan Humphrey. Also, who could forget the ménage à trois she has with Dan and Vanessa?!
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On the season 3 episode entitled "Dan de Fleurette," Tyra Banks portrayed Ursula Nyquist, an anxious actress who befriends Serena after she's hired as her assistant.
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There once was a time when "Harry Potter" and "Tenet" actress Clémence Poésy appeared on "Gossip Girl." During the show's fourth season, she played Eva Coupeau, Chuck's girlfriend who saves him and nurses him back to health after he's shot.
It wouldn't be "Gossip Girl" if there wasn't a forbidden romance! During the second season, "Twin Peaks" and "Riverdale" star Madchen Amick played Catherine Beaton, an older woman who has an affair with Nate.
On the show's fourth season, "Arrow" actress Katie Cassidy played Juliet Sharp, a woman who aims to take down Serena as she believes the beauty is the reason her brother, Ben Donovan, was sent to prison.
On season two of "Gossip Girl," another "Arrow" alum — Willa Holland — portrayed Agnes Andrews, a wild model who befriends and then backstabs Jenny Humphrey.
Broadway and "Graceland" actor Aaron Tveit played the recurring role of Trip van der Bilt, Nate Archibald's cousin, during seasons 2, 3 and 5. Trip, who eventually goes into politics, competes with Nate for both the affections of their grandfather and Serena.
We love a good flashback episode! Before she joined the "Pitch Perfect" film franchise, Brittany Snow appeared on a season 2 episode of "Gossip Girl" entitled "Valley Girls." She portrayed a young Lily van der Woodsen who ventures to Los Angeles to reunite with her sister.
As for who Lily's sister? She was portrayed by Krysten Ritter! The "Jessica Jones" actress starred as Carol Rhodes, Lily's rebellious older sibling who foregoes her life of luxury for independence.
Kristen Bell arguably had one of the most important roles on the original "Gossip Girl" — she was the show's narrator! On the series finale in 2012, she portrayed herself alongside another familiar face…
Rachel Bilson shared the screen with "Frozen" voice actress Kristen Bell on the "Gossip Girl" series finale. Rachel is seen auditioning for a part in Dan Humphrey's movie adaptation of his book.
Gwen Stefani's appearance was short but sweet! On the season 2 episode "Valley Girls," the No Doubt singer was featured as the frontwoman of a fictional rock band called Snowed Out.
Another female artist who made a cameo during a "Gossip Girl" episode? Florence Welch of Florence + the Machine. The band performed the track "Cosmic Love" on the season 4 episode titled "Panic Roommate."
From "Project Runway" to his latest endeavor with Heidi Klum, "Making the Cut," Tim Gunn has become a household name in fashion reality TV. He appeared as himself during a season 4 "Gossip Girl" episode and consequently labeled Taylor Momsen a "diva" after sharing the screen with her. Yikes.
On the season 4 premiere episode titled "Belles de Jour," model Karlie Kloss made a cameo as herself at one of Lily van der Woodsen's parties.
Mother Monster! During the show's third season, Lady Gaga performed her hit track "Bad Romance" as part of a performance art piece on the show.
On the season 2 episode "Bonfire of the Vanity," pop star Cyndi Lauper made a cameo as herself during Blair Waldorf's 18th birthday party.
Short but sweet! Olympic gymnast Nastia Liukin made a brief appearance during the episode "It's a Wonderful Lie" during season 2. She was seen flirting with Chuck Bass.
On a season 4 episode, businesswoman and former presidential adviser Ivanka Trump played herself alongside husband Jared Kushner as the host of a New York Observer Party.