The hunky lad will release the charity single before Red Nose Day on 14 March.
But no one knows what his song will be, whether it will be a cover, or if he'll be joined by showbiz mates on it.
Other pop idols who have sung the song in previous years include the Spice Girls and Westlife.
The Irish boys scored a number one hit two years ago when they did a cover of Uptown Girl.
And Gareth's former rival and Pop Idol winner Will Young has done his bit for charity already.
He recorded Don't Let Me Down, especially for Children in Need.
Comic Relief Day is officially the time when people do mad things like bathe in beans to raise dosh.
And the money raised from Gareth's single will go to the charities Comic Relief helps.