“Gossip Girl” star Michelle Trachtenberg cozied up to John Mayer at Soho House in Los Angeles the other night. “Michelle was bouncy and flirty. She seemed smitten with him,” said one witness. But the singer/guitarist “seemed too cool for school and non-expressive.” A flack for Mayer had no comment. Trachtenberg’s rep said the pair chatted for five minutes and then she resumed dinner with her family. “They’ve known each other for years,” the rep added.