Natalie Portman will show her support for her new squeeze, New York City Ballet principal dancer Benjamin Millepied, by gracing the company’s spring gala with her presence Thursday at the David H. Koch Theater at Lincoln Center. Millepied, the choreographer on Portman’s new movie, “Black Swan,” is creating a world-premiere ballet for NYCB this spring. Sources say he and Portman are “very much in love.”