Hadid, 25, is expecting her first child, a daughter, with partner Zayn Malik, 27.
Real estate developer Mohamed, 71, posted a touching handwritten message to the baby on Instagram.
“Hello little grandchild, it is me, my heart as happy as can be,” he wrote.
“I wish for you the sun & moon, I wish for you a happy time. Know that grandpa’s always here, I’ll do anything, anything for you, my dear.
“When I heard you were on the way, I smiled and wiped a tear away. I cried the tear because I knew my heart would always belong to you.”
Mohamed ‘s post sparked fan speculation Hadid had given birth, but he later clarified the baby had not arrived.
He is not the only member of the family sharing their excitement on social media. On Tuesday Hadid’s model sister Bella, 23, posted a throwback picture of them together.
It showed a smiling Hadid looking to the sky and cradling her baby bump, while Bella struck a similar pose.
She wrote in the caption: “June 11, 2020 ..two buns in the oven except mine is from my burger and gigi’s is from @zayn. I love you both so freaking much.”
Bella added that she “can’t stop crying”.
Hadid confirmed she was expecting in April, telling chat show host Jimmy Fallon she and former One Direction singer Malik were “happy and grateful for everyone’s well wishes and support”.
Los Angeles-born Hadid and Bradford-born Malik have been dating on and off since late 2015.