Barlow has been highly critical of the Northern Irish teen in recent weeks, but the singer said that she appreciated the Take That star's remarks.
"I don't think Gary's been harsh. If you're not critical of someone, how can they grow? And I need to grow," Devlin told Digital Spy at the Twilight: Breaking Dawn premiere.
The 17-year-old went on to reveal that she would be singing a "well-known romantic song" for Movies Week. "But it's not a slow song," she added.
Devlin also responded to reports that Louis Walsh may leave the ITV1 reality show at the end of the year, arguing that the series "couldn't live without him".
"I know people have a giggle at him, but he's the light-hearted side of the show. He's a lovely guy," she added.
Additional reporting by Tom Mansell
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