"Expecting a baby girl this year to round out our little gang," she captioned the happy picture.
As expected, hundreds of congratulatory comments quickly began rolling in, with fellow Aussie models like Nicole Trunfio, Lara Bingle, Jessica Gomes, Megan Gale and Georgia Fowler expressing their excitement for the soon to be mum-of-three.
"Gorgeous!!! Congrats 💖," Megan Gale wrote.
"Oh darling - congratulations! 💖," fashion designer Brooke Testoni commented, to which P.E Nation founder Pip Edwards simply replied, "🙌🏻🙌🏻🙌🏻"
The new addition marks the third child for the loved up couple, who are already parents to their six-year-old daughter, Naia, and a three-year-old son, Jet.
"I think it's all about teamwork. You have to make sure everyone is on board and obviously the kids come first, but work is pretty important too when you have an obligation to provide for them," she said.
Gemma famously dated Australian actor Health Ledger and was reportedly in a relationship with the 10 Things I Hate About You star at the time of his accidental drug overdose in 2008.