The 100th episode of the series, set in Manhattan, is due to air in the U.S. on Monday (30Jan12), and to mark the milestone Bloomberg headed to the Silvercup Studios set on Thursday (26Jan12).
Stars including Blake Lively, Penn Badgley and Kelly Rutherford met the mayor, and he's delighted with the interest they've generated in the Big Apple.
He says, "Here's a hot tip for Gossip Girl: Spotted - 100,000 New Yorkers making a living from the film and television industry. Could 2011 have been the City's busiest year ever for television production?
"We're proud to be home to employers like Gossip Girl, Warner Bros. and Silvercup Studios, and I want to congratulate Gossip Girl on its 100th episode and nearly five seasons of generating jobs and drawing fans to New York."
And the cast was overjoyed with Bloomberg's approval - in a post on Twitter.com, actor Matthew Settle writes, "The mayor was funny. Pretended to be a casual GG fan but then knew all this detail about the show!", while Kaylee DeFer added, "Mayor Bloomberg just honored us with our own day! Happy Gossip Girl Day!"