At about 05:20 central time, a Beech 58 on a Part 135 charter flight landed gear-up at Gulfport-Biloxi International Airport. The pilot and two occupants were uninjured. The flight departed the airport and the cabin door came ajar. The pilot landed and closed the door. The flight again departed and the cabin door popped open again. When the pilot returned for another landing, he neglected to lower the landing gear.