Gwyneth, 41, posted a picture of herself in costume, dressed in a black and white checked skirt, light blue T-shirt and black ankle boots. A second snapshot showed she was also wearing pendant spelling out her character's name Holly.
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Finally, the actress shared a selfie with Glee co-creator Ryan Murphy, which shows the pair posing and puckering up for the camera with the caption, "me x ryan murphy (lips) #glee."
Gwyneth wasn't the only star to post a picture from the set. Lea Michele, who plays Rachel Berry in the hit series, posted two photos, including one with Dianna Agron, as Quinn.
Their on-screen reunion is particularly exciting for Glee fans. Dianna last reprised her role at Will and Emma's wedding in season four, but was invited back to be part of the 100th episode, with Chace Crawford confirmed to play her love interest.
"Early 80s glam @Glee today with @DiannaAgron," Lea tweeted next to a glamours photo of the pair, to which Dianna replied, "Want to dress like this every day."
Lea's second snap shows her in the same outfit, with the caption, "American Hustle #Glee."
The 100th episode has been split into two parts and will be aired on 18 and 25 March. Series six of the hit musical show hits screens on 25 February.