After the British boy band split in 1996, Gary saw his weight shoot up to almost 17 stone, as he admits he ate anything he could get his hands on.
The 40-year-old has now brought his weight below 12st thanks to eating less and exercising more.
He told the Radio Times: 'Now I'm under 12 stone. Eight years ago I was 16 stone 11 at my heaviest.'
When asked how being overweight made him feel, the X Factor judge said: 'Horrible. For someone so big, I felt incredibly small. It was about food, obviously, because I was shoving it in my mouth, but it was more a reaction to who I'd been.
'I'd decided: 'OK, nobody wants me but I don't want to do it anyway, and to make sure I don't do it again, this is how I am going to look.'
'I wasn't saying it in my head, but I realised afterwards that it was a form of depression.'
He added: 'I'd always played music since I was 10 and then I'd gone through this rollercoaster ride of fame and I just couldn't believe that it was all over and I was only 24.'