The two musicians were married from 2002 to 2016 and welcomed three sons during their time together: 17-year-old Kingston, Zuma, and nine-year-old Apollo.
In honor of Zuma's big day, Gwen, 53, took to Instagram with a compilation of memories, capturing his growth through a series of photographs, which you can check out below...
"Happy Birthday Zuma" was splashed across the video in big blue letters, spanning through many of his moments with both his mom and his step-dad Blake Shelton.
"Happy 15th birthday Zuma!!! We love you more than anything," Gwen sweetly wrote alongside the video, and fans couldn't believe how much he'd grown.
"Wow 15 already!? Happy birthday," one fan commented, with another adding: "Can't believe how grownup he's become, such a handsome big boy and very like mummy," and a third writing: "Wow…15!! Zuma you are a cutie Happy Birthday Zuma! Have an awesome day!"
Gwen and Gavin divide their time co-parenting between the United States and Europe, with Gwen spending a majority of her time with Blake at their Oklahoma ranch and Gavin in the United Kingdom. They both often work and perform in California as well.
While appearing on the podcast Not So Hollywood with host Adrianna Costa back in June, Bush frontman Gavin shared inside details of what their dynamic is like as co-parents, and how the boys' step-father Blake figures in.
"I think you can go one of two ways," he said. "You can either do everything together and really co-parent, and see how it goes. Or you can just parent. I think we just 'parent'. We're really different people, I don't think there's much similarity in the way we bring them up."
Gavin credited their differing ideas on life and parenting for the diverse upbringing their three sons received. He continued: "I think that gives them an incredible perspective to then choose which pieces of those two lives they'd really like to inherit and move on with, and which part comes out of the whole process.
"Because that's what's important, is to give them a wide view of things. We definitely have some particularly opposing views and it'd be really helpful for them to make their own minds, as they should, as individuals."
The 57-year-old rocker added: "I just know that wherever they are, in whichever house they are, they're loved and supported. And that's really what it comes down to, trying to help them realize what they want. The main thing is I'm really connected to them, that's what I want. It'd be so sad to be any other way."
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