The 40-year-old Take That star is helping chubby contestant Craig Colton to lose weight.
Colton, 22, who works in a biscuit factory, told UK newspaper The Sun: 'I mentioned to Gary that I wanted to lose weight, so he put me on this diet.
'It's strict, but I'm full of energy. I lost a stone in a week and more than a stone since being at the judge's house stage in Los Angeles.'
Fellow wannabe Marcus Collins, 23, added: 'He has given us a healthy eating plan. It is smoothies for breakfast and slow-releasing energy – snacking on almonds instead of sweets. He also advised us to work out.'
Barlow revealed earlier this week how he suffered from depression after his weight hit almost 17st following Take That's split.