The singer - who turned 40 in January - mocks the eldest member of the boy band, Howard Donald, 43, about his advancing years, and he admits it gets to him.
Howard said: "I am very sensitive about my age. Gary will make jokes that I'm 46 or 47 and it really p***es me off. I feel like I'm still fit enough to go on, I don't feel my age."
While Howard is touchy about his age, Gary insists getting older doesn't bother him and is proud the group - which also consists of Mark Owen, 39, Jason Orange, 40, and 37-year-old Robbie Williams - are still going strong.
He said: "People still find it hard to believe I'm 40. Actually I'm not bothered about age. I'm really happy and it's lovely to be 40, I'd recommend it to anyone.
"Music is a young person's industry but you want a few of the older bands like us around. We don't have a plan for how long it is going to last."
To prove they're not past it, the group are set to embark on a sold-out European stadium tour this summer, featuring energetic dance routines and a £15million stage show which is rumoured to run to two hours in length.