"None of us were sure if he was telling the truth, so we just carried on. Then a few weeks later we were away for the weekend, and took a trip to the local supermarket. We went down the cereal aisle - and there was my face on the Weet-Bix box. We had honestly thought Granddad was joking."
After honing her rugby skills at boarding school in her teens, Goss then concentrated on making the sport her career.
It culminated in a successful 2016 campaign where she was named New Zealand Player of the Year, and was a finalist for International Player of the Year.
But she hasn't forgotten her Weet-Bix days, saying she still has her TRYathlon medals at her parents' house.
"My parents still have the picture of me on the box too!" she laughed.
Goss is now an ambassador for the Weet-Bix TRYathlon, encouraging kids to get involved in the 16 events around the country this summer.
She says the event provided a great foundation for her career.