J.R. Gaines Jr., vice president of Superconductive Components Inc., has resigned to be vice president, corporate development, for Oak Ridge Micro Energy Inc. in Salt Lake City. His resignation was effective April 30.   . . .   BAE Systems Advanced Systems has promoted Vice President Patricia (Pat) McMahon to general manager at its Greenlawn, N.Y., facility. In addition to her current program management and marketing responsibilities at Greenlawn, McMahon will now oversee the unit's engineering, operations and quality functions, giving her responsibility for direct execution of Advanced Systems, Identification Friend or Foe and Display Systems businesses.