The duo are currently dating - but the 20-year-old model has admitted they may never have got together were it not for 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.''
Meanwhile, Gigi said Joe first asked her out when she was 13 years old - and she rejected him.
The stunning 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'' about the prospect of dating him.
Speaking in a Periscope Q&A, Gigi 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.''