The presenter, who joined the ITV2 spinoff after Konnie Huq was axed, revealed that the Take That star is the only judge yet to show emotion on the singing competition.
"He's still the only judge who hasn't cried yet, so me and Olly [Murs, co-host] have made it our mission to make Gary cry," Flack told Fabulous magazine.
Speaking of her rumoured romance with Prince Harry, she added: "Harry's a good guy, but there was never anything romantic between us. I'm hanging up my party shoes, I'd much rather have a night in with a film and a bowl of lettuce."
Flack is also a team captain on new ITV2 gameshow Minute To Win It.
Watch the Xtra Factor trailer below: