No, you do not gain U.S. citizenship if you are selected a winner in the Green Card Lottery. What you get is U.S. permanent residence. You can request U.S. citizenship after living in the United States for 5 years.
People applying to become U.S. citizens must meet several requirements.
Applicants must also get a passing score on a simple citizenship test. This is a 10-question oral test and the applicant must answer 6 questions correctly to pass.
One exceptional way to become a U.S. permanent resident is to apply to the American Diversity Visa (DV) lottery program, which allows hopeful U.S. immigrants to apply for a drawing to become a permanent resident.