Expectant mothers can usually find out the sex of their baby between the 16 to 20-week mark, and it's believed Gigi is currently 20 weeks pregnant.
Neither Gigi nor Zayn, 28, have spoken publicly about the pregnancy rumours, but a source said this week they are "very excited".
The insider claimed: "Gigi and Zayn are expecting their first child together and the couple is very excited! Gigi has kept the secret close to her family and friends for a while, as she's only a few months along. Once Gigi and Zayn got back together at the end of last year, it was like they never skipped a beat and knew what they had was special. The couple and their families are overjoyed."
Meanwhile, Gigi previously hinted at the possibility of starting a family with the former One Direction star, as she said she's looking forward to her future outside of modeling.
She said: "I think that as I get older ... well one day I'll start a family and I don't know if I will always be modelling. I love the creative side of fashion, it's so fulfilling. The people I work with make me so happy, I'm so lucky to be around them. But who knows? Maybe I'll take up full time cooking!"