//metrognome logo// LAURA GILBERT was proud of herself for losing those extra five pounds. But little did she know that her successful diet would directly lead to her being branded a criminal - and put her in the middle of a battle that could forever change New York. Here's the story: Last month, the newly trim Brooklynite packed up her ill-fitting clothing and joined two friends who were having a stoop sale on a Williamsburg side street. The sale went fine, as Gilbert scored $37 for clothing she wouldn't have worn again anyway. But just as the three friends were calling it a day, an officer from the 90th Precinct handed them a summons for not having a peddler's license. "Since when do you need a peddler's license to have a stoop sale?" Gilbert asked. In fact, you don't. According to the city Law Department, people can sell anything, as long as they are within three feet of their homes. Drift beyond three feet - as Gilbert and friends did - and you're no longer having a stoop sale, you're selling goods without a license. Well, Gilbert is fighting back. On Thursday, when she is scheduled to appear in Brooklyn Criminal Court, she intends to plead not guilty and force the first-known test case of every New Yorker's unalienable right to sell old clothes, kitchen utensils or even vinyl copies of "Frampton Comes Alive!" whenever and wherever they want. Maybe it's not the trial of the century, but Laura Gilbert - thanks to a pair of low-rider culottes that just wouldn't fit right anymore - is fighting for all of us. "If what we did is illegal, it's stupid and unfair," said Gilbert, employing fairly sophisticated legal jargon. "People need to be able to unload their crap." A source at Brooklyn's Community Board 2 - which covers stoop sale-filled Brooklyn Heights and Cobble Hill - recommended that the police take a "laissez-faire attitude" toward stoop sales "as long as no one is blocking a major sidewalk." That won't help Gilbert and friends in court, but it's clear that millions of stoop-sale lovers are behind them in the court of public opinion. Besides, Gilbert feels she's already been punished for her "crime." "I got a vicious sunburn that day," she recalled. "Isn't that punishment enough?" --30-- gersh.kuntzman@verizon.net