A LONG DAY'S NIGHT: Longchamp kept even celebrities hungry at a dinner inside its SoHo store Thursday night, not to mention the other guests who sat waiting for a morsel or two. "I'm starving," said Maggie Gyllenhaal. Seated around several square tables, various editors and friends of Longchamp gathered to toast DJ Michel Gaubert's new accessories collection with the firm — and until about 10 p.m., toasting was all they did. Too bad Gyllenhaal didn't get the memo to show up hours late like her other fellow celebrity guests, such as Leighton Meester of "Gossip Girls" and photographer Mick Rock, but the actress was in good spirits anyway, talking about her recent advertising campaign with Agent Provocateur. "It was a great experience," said Gyllenhaal. "But my brother once called me from Heathrow [Airport] to tell me he had passed a giant picture of me in my underwear. I guess I forgot that would happen."