The recipe is working. In her age category, Gunthorp holds the New Zealand record for 100m, 200m, 400m and 800m freestyle, 50m and 100m backstroke, 200m and 400m individual medley and 100m and 200m butterfly.
At the World Masters Games, she hopes to add the 200m backstroke to her list, before taking aim at the equivalent short course records later this year.
Gunthorp took up swimming as a nine year-old at Parnell Baths in 1956. She showed potential, and three years later transferred to the Olympic pools in Newmarket, where she was trained by the renowned Jack Lyons.
Lyons' most famous alumni was Philippa Gould, who set four world records in the late 1950s, and he also coached many other Olympians.
Gunthorp progressed well, and probably should have gone to the 1966 Empire Games.
"I qualified for the 200m backstroke but missed the time for the 100m backstroke by a whisker," said Gunthorp. "It's different now but in those days, they didn't take you unless you qualified for both events. The girl who went [Margaret Macrae] got bronze in the 200 so I did wonder what might have been."
Soon after that, Gunthorp gave up the sport.
"I was 18 and I stopped swimming. There was no longevity in the sport back then - by that age you were a bit of a has-been."
Swimming was confined to history as she focused on raising her family, encouraging her two daughters into their various sporting pursuits.
She barely set foot in a pool for more than three decades, before a move to Australia reignited her interest.
"I moved to Queensland with my partner in 1999 and was looking for things to do," said Gunthorp. "Swimming is such a big part of the culture there so I went down to the pool and started doing a few laps."
Her talent was recognised, and she was encouraged to join the Miami Masters squad, which took her to meets all across Australia. When Gunthorp and her partner moved back to New Zealand in 2004, she continued swimming.
She'll swim up to 15km a week, and spends much of the rest of the day in the pool, teaching all ages (from pre-school to adult) as an instructor for SwimMagic.
But over the next two weeks she'll be Super Gran again.
Gunthorp's main focus this year is the 2017 Fina World Masters Swimming championships in Budapest in August, but she will enjoy competing on her own turf. She'll swim in five individual, four relay events and the 2.5km open water swim.
