“I was in a room full of dicks today,” Lady Gaga announced after spending part of the day working as a day trader at BGC, the brokerage firm Cantor Fitzgerald rebuilt following the World Trade Center attack, raising money for its annual celebrity charity event. Clad in a demure — for her — YSL pantsuit, Gaga worked the phones, singing “Poker Face” to floor traders in exchange for more money and then telling them she loved them. “This was a very intense environment, and I was okay,” she said. “We did some pretty massive trades.” Gaga was raising money for Mercy House, a charity that supports young women and families in the South Bronx. The charity is close to her heart, she said, partly because her best friend from high school runs it. “She was the girl in school who, no matter how many girls made fun of me or didn’t get my eccentric style or my ambitious personality, she always stood by me.” Plus, it doesn’t hurt to take a break out from Fashion Week: “Vanity is only skin deep,” she said.