Chuck, played by Londoner Ed Westwick, gets his haircuts at Piccadilly.
Blair compares Chuck to House. Leighton Meester guest-starred on House, playing a teenager with a huge crush on the title character. Coincidentally, Chuck and House are both dark, twisted Jerk with a Heart of Gold type characters, and Ed Westwick and Hugh Laurie are both Englishmen while their characters are American.
What makes it funnier is the scene where Chuck is talking to Blair about her British boyfriend and says: "You can't tell me Bertie Wooster is satisfying your needs."
Blair mentions herself and Chuck role-playing "Wuthering Heights". Ed Westwick had been cast to play Heathcliff in a new remake but was dropped not long before the episode where Blair makes the mentioning aired.
As a side effect of the show's Idiosyncratic Episode Naming, season four's "The Townie" references The Town in which Blake Lively has a role. The same season also saw "Panic Roommate" mash up Panic Room and The Roommate (Leighton Meester plays the title role in the latter), and "Easy J" riff on Easy A (in which Penn Badgley appears). The first season also has an episode called "Blair Waldorf Must Pie!" (from John Tucker Must Die, which also has Penn Badgley) and the Series Finale is called "New York, I Love You XOXO" (after New York, I Love You, with Blake Livelynote and Rachel Bilson, who has an uncredited cameo in the episode).
In Season 1 when Jenny is trying on dresses at Blair's house, Blair tells her a dress is too Hannah Montana. Taylor Momsen was one of many actresses considered for the role.
During the build up to and actual Blair's wedding, Wallace Shawn makes at least two Princess Bride references by name.
In Season 6, Chuck tells Nate that maybe he needs to find himself a Hot Tub Time Machine. Former guest star Sebastian Stan (Carter Baizen) starred in this movie.
In the final season's "It's Really Complicated," when Serena tells Blair that her ex Steven (played by Barry Watson) tried to get back with her, Blair (who'd rather it was him than Dan again) replies "Ooh, you must be in seventh heaven!"
Dawson Casting: All the main "teen" cast, with the exception of Jenny (Taylor Momsen was an actual teenager when cast) apply into this trope. Particularly notorious is Nicole Fiscella (Isabel Coates, one of the mean girls), who was born in 1979.
A new example for season 6 is Sage who is 17 played by Sofia Black D'elia who was born in 1991, two calendar years older than Momsen.
Dyeing for Your Art: Leighton Meester dyed her blond hair brown before her audition, and kept it brown whenever the show was filming.
The awful dreads they put on Jessica Szohr have to count.
Enforced Method Acting: Billy Baldwin refused to meet Blake Lively before their first scene together because he wanted the experience of seeing your father/daughter again after 14 years to be as real for both of them as possible. He also stated that he pretended Blake was his real life daughter.
Fake Brit: Lord Marcus, played by American Patrick Heusinger. Making it an American playing a Brit who fakes being American. Add to that the scene where Chuck (played by a Brit, as noted above) imitates Marcus' accent to pull a Bed Trick on Blair. A Brit playing an American faking a British accent as done by an American.
Mean Character, Nice Actor: Leighton Meester is by all accounts a lot easier to take than Blair Waldorf (which admittedly wouldn't be hard).
Name's the Same: Nate Archibald is also the name of a former NBA player (wikipedia searches redirect to the player).
Real-Life Relative: Jessica Szohr (Vanessa) and Ed Westwick (Chuck) were dating as well (though, in a bit of a twist, Westwick discovered that Szohr was seeing other men), even their fictional counterparts don't get along very well.
Leighton Meester (Blair) and Sebastian Stan (Carter) (broken up).
Blake Lively (Serena) and Penn Badgley (Dan) (broken up).
The Red Stapler: Chuck's trademark scarf sold out within days of the pilot episode airing.
Romance on the Set: Blake Lively (Serena) and Penn Badgley (Dan), Ed Westwick (Chuck) and Jessica Szohr and Leighton Meester (Blair) and Sebastian Stan (Carter). All have now broken up.
Stillborn Franchise: 2008 saw the release of OMFGG – Original Music Featured on Gossip Girl No. 1. There would not be a No. 2 (the CD didn't even include the show's opening/closing music for some reason).
Throw It In: This ad-libbed dialogue in the last scene from season two:
Blair: (after Chuck told her he loves her) But, can you say it twice? No I'm serious, say it twice.
Chuck: I love you. I love you. That's three. Here's four - I love you.