Zak Khan wasn’t always the Gyro King of Brooklyn. He started out with a little halal cart down on Wall Street, in the Financial District, just after the September 11 attacks. But running a halal cart is a rough business: Permits cost an arm and a leg, the city fines you without mercy, and in the end it was actually cheaper for him to open a brick-and-mortar restaurant. He expanded steadily , and today his gyro kingdom spans the steppes of Central Brooklyn , from Midwood and Bay Ridge to Coney