The 20-year-old model was pursued by the singer, 26, at the Grammy Awards six years ago but she turned down his offer to go a baseball game because she was ''so nervous.''
Speaking in a Periscope Q&A, she said: ''We met at the Grammys when I was 13 years old and he asked me to a baseball game, and I said, 'No.'
''I was so nervous; I literally didn't even know what it meant to hang out with a boy.''
Hinting the handsome star didn't know how young she was at the time, she added: ''And also, Grammys are on a Sunday, and I didn't want to tell him that I had school the next day, so I was like, 'No, maybe next time.'''
But the rejection didn't put the former Jonas Brothers star off as he scribbled down his number on a piece of paper and handed it to Gigi's famous mother Yolanda Foster.
The blonde stunner explained: ''[He] wrote it [his number] on a piece of paper and gave it to my mom.
''And then we've been friends ever since. Except now. We're more than friends, obviously.''
The couple first sparked rumours they were an item in May after they stepped out in Los Angeles together, just days after Gigi had split from boyfriend Cody Simpson, 18.
Over the past four months, the good-looking pair have been inseparable and regularly put on a very public display of affection while out and about in busy cities.