Here's Leslie's original vocal track, (she was dubbed by Betty Wand in the released film).
This track fit in easily, since it is the actual track used for filming. Here's how it worked, according to Hugh Fordin's "The World of Entertainment!":
Out of necessity, the prerecordings for "Gigi" were made in a completely unorthodox and lopsided manner. [Performers] recorded their numbers [while in Paris] to Andre Previn's piano accompaniment. Voice and piano were recorded on separate bands and Lela Simone made sure the voices didn't leak on to the piano track and vice versa because ultimately the full orchestra would be added to the voice tracks at Culver City. .