Trachtenberg, from New York City, played young socialite Georgina Sparks of Manhattan's Upper East Side who created havoc throughout the show, blackmailing Serena van der Woodsen (Lively), among other schemes.
Fellow Gossip Girl star Ed Westwick, who played Chuck Bass on the show, said he was "sending prayers" in a post to his social media, featuring a picture of Trachtenberg as Sparks.
Emma Caulfield, who starred as Anya Jenkins in the supernatural drama, said the cast had "lost a little sister" in a post shared to her Instagram story where she referred to the actress as "Mish Mish".
Michelle Trachtenberg had a "huge heart" and was "fiercely loyal" to friends, actress Blake Lively said.
The Buffy The Vampire Slayer and Gossip Girl star died aged 39 on Wednesday after police were called to a luxury apartment complex on Central Park South, New York City.
