Newt Gingrich has a problem with John Lewis, the Democratic congressman from Georgia, and about 100 of his friends. Just as the House Speaker was all set to kick offhis campaign to reform Medicareat an Atlanta hotel this morning, in marched Lewis, a civil rights-era veteran, and four busloads of demonstrators from the Service Employees International Union and other labor groups. When Gingrich responded by rescheduling his speech for later in the day, the troops' cries of "Where is Newt?" turned to chants of "Newt is scared!" The Speaker turned up a few hours later to condemn the "mindless protest," then explained that allsenior citizens will keep their Medicare even after GOP cuts. Of Lewis, who today likened the struggle over Medicare to the civil rights march on Selma, Ala., 30 years ago, Gingrich said: "He has to decide whether he's a protester or a congressman.