If Gilles Mendel, who created the costumes for the new Peter Martins' number at the New York City Ballet's Spring Gala last night, had a case of nerves, he sure wasn't showing them. "This is like a total vacation for me," he said as he arrived at Lincoln Center. "But, uh, I did just lose the button to my jacket and if I'm supposed to go on stage and take a bow, I'm not sure what I'll do."