The 40-year-old will be mentoring one of the categories at the judges houses stage, where he will bring the contestants to his home to decide who to take through to the live shows.
But he has warned the young boys on the show that he doesn't want any of them trying to impersonate his Take That co-star Robbie.
According to UK newspaper Daily Star, Barlow had a message for his group: 'No boozing, take this seriously or you're out. I'm very excited about tomorrow so good luck to you all and please get a good night's sleep.
'I know the pressure's horrible but try and enjoy it.'
A source added: 'Gary is taking his job as a mentor seriously. This show is the best opportunity they will ever have to crack the music industry.'