ST. LOUIS, Mo. (AP) — Ann Gibbs of suburban Glendale went right to the top with her premarital problems — to Pres ident Johnson himself. Fearful that her airman france would not be given a leave for their wedding March 5, she wrote to President John son and to Sen. Stuart Syming ton, D-Mo. Miss Gibbs said Monday that the Pentagon called her last week with word that her fiance would be given a leave in time for the wedding Her fiance, Stephen Schneider of Waterloo, Ill, is undergoing basic training at Lackland Air Force Base, Tex