The 28-year-old star took to the stage at the Dolby Theatre in Los Angeles to sing 'Climb Every Mountain' and was reduced to tears when the legendary actress emerged from behind the curtains.
Speaking to E! News, Gaga said: ''I started crying. It was a beautiful moment. As a child performer and a music lover, you dream your whole life that you will get to perform for somebody like that and it was just magical.''
The 'Applause' hitmaker went on to reveal she had no idea Julie, 79, was going to join her on stage for the song and it is a moment she'll now cherish forever.
She explained: ''I had just spoken to Julie on the phone the day before and she was telling me to just relax and enjoy myself and then when she came out at the end, it was the perfect moment. I can't even explain it. It was a dream come true. It was a surprise.''
Meanwhile, the eccentric singer raised eyebrows when she turned up at the glamorous event in a huge ball gown and red rubber gloves, which covered her heart-shaped diamond engagement ring that her partner Taylor Kinney had recently proposed with.
The couple got engaged on Valentine's Day (14.02.15) in Manhattan, New York, following three years of dating, but she admits they're in no hurry to tie the knot.
Asked if she's made any wedding plans yet, she replied: ''We're just excited to be engaged.''
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