Gareth, now 29, enjoyed a fling with Katie in 2001, shortly after he came second in TV show Pop Idol and while she was pregnant with her first child Harvey.
But when the pair were rumbled, 17-year-old Gareth denied it - and his lies just snowballed. The singer even threatened legal action, before admitting a year later he had slept with her.
Speaking to The Sun, ahead of his appearance on The Big Reunion with newly-formed boy band 5th Story, Gareth has said he regrets telling fibs.
He told the newspaper: "I don't regret sleeping with Jordan. It was an amazing time. I wish I hadn't lied about it though."
In 2009, during an episode of Piers Morgan Life Stories, Katie - who is now married to third husband Kieran Hayler and has four children - said she did regret sleeping with Gareth.
She told Piers: "It's terrible - I was still young, I was pregnant, I didn't have the dad about and I still wanted to be loved. It was wrong when I look back. It was over in seconds!"
Gareth and his new bandmates - Dane Bowers, Kenzie, Adam Rickitt and Kavana - will appear in the third episode of the new Big Reunion series.
It is thought he will talk more about his fling with Katie, while the other singers recall their own highs and lows.
The Big Reunion kicks off tomorrow night (6 February) and the first episode tells the stories of Damage and Girl Thing.
The Big Reunion, 9pm, ITV2, Thursday 6 February.
Watch our behind the scenes video with 5th Story here:
Check out the Big Reunion stars in before and after photos:
Copyright: Rex Features