The Victoria's Secret model reportedly told Zayn that things were moving too fast.
Life & Style magazine (via Daily Mail) quoted a source close to the couple who said Gigi "didn't feel ready" for him to put a ring on it.
"She's only 21 and doesn't feel ready to tie the knot, so she turned him down," the source said, adding that her mother's marital woes played a factor into her decision. "She's seen her mom go through two messy divorces, so Gigi wants to make 100 per cent sure Zayn's the one before she makes a lifetime commitment."
Yolanda Hadid, a former "Real Housewives of Beverly Hills" star, announced she had split with David Foster in December 2015. She divorced Gigi's father, Mohammed Hadid, in 2000.
Zayn and Gigi began dating in November 2015, having gotten together after he split with his fiance Perrie Edwards, who he was engaged to for two years. He and Gigi briefly split over the summer, but quickly got back on track.
"Right now, it's back on, but it could be off again in an hour," a source told Us Weekly at the time. "They are young and keep going back and forth. Things are testy between them."
Around the same time, the former One Directioner said he was experiencing the "worst anxiety" of his career. Gigi took to social media to support him.
Addressing her public note to "@zaynmalik ♥," the model began, "Z - I've seen the battles you go through and the way you fight to get to a place that allows you to get up there for your fans."
She continued, "Your bravery in those times makes me proud, but your honesty last night proved what you're all about, being real. Human recognizes human. You made the best of the situation and have given your fans an opportunity to understand you better as a performer."
Her note concluded, "Those who can find compassion now are the ones that deserve to watch you continue to grow. We are all here to support you and make each experience easier. Your talent and good heart will never lead you wrong. Love you and so proud of you always :) xG."