The first six-month window for Americans to gain health insurance under the Affordable Care Act closed on Monday with large numbers of consumers speeding to get coverage at the last minute. Some of them encountered obstacles as HealthCare.gov, the main enrollment Web site, faltered on and off throughout the day. Union halls, shopping-mall kiosks and insurance company lobbies across the country were jammed with people racing to get insurance on the final day before the law required most Americans to choose health insurance or risk a financial penalty.