He told Radio 1: "Wow. I'm over the moon. Will is a really good friend and we're both happy so that's cool."
He added the Pop Idol tour was going really well and he was working hard on his first album.
Gareth has also thrashed long-standing artists like George Michael, who went in at number seven with Freeek!, and popular boy band Blue, who charted at number six with Fly By.
Gareth, who narrowly missed out on winning TV's Pop Idol, has sold more than 850,000 copies of his debut single in what's been dubbed 'Garethmania'.
In other chart news, Marilyn Manson entered the charts at number five with his cover of Tainted Love, and Will Young's still at number two with Evergreen and Anything Is Possible.