Katy Perry unmasked as Met Gala's mystery guest.

The ET singer walked the steps at New York's Metropolitan Museum of Art for the Costume Art themed gala on Sunday night. In keeping with fashion's night of night's Fashion Is Art dress code, Perry opted for a white, strapless Stella McCartney gown featuring a long train and white, elbow-length gloves. But it was almost impossible to figure out who she was, under a white and silver mask reminiscent of an astronaut's helmet. The star-studded event, which was co-chaired by Beyoncé, Nicole Kidman, Venus Williams and Anna Wintour…