The pilot reported 'the aircraft engine stopped' and he made a 'forced landing.' The forced landing was made in a level gravel field. During the landing, both main landing gear collapsed, and the left gear penetrated the upper surface of the wing. The propeller was positioned horizontally and was undamaged. The left fuel tank contained 18 gallons of fuel, and the right fuel tank was empty. The fuel selector was found in the off position. The engine operated normally during a postaccident test run.