The Take That singer revealed in February that he is to become a father for a fourth time.
"I have more news .....," he tweeted.
"There's gonna be another Girl in the Barlow household ! (sic)".
The couple, who married in 2000, are already parents to Daniel, 11, Emily, 9, and 3-year-old Daisy.
Barlow is the first of three Take That members to be expecting a child this year, with Robbie Williams to become a father for the first time and Mark Owen to welcome his third child later in the year.
Gary Barlow has also confirmed that he will return as a judge for this year's X Factor.
> Simon Cowell 'wanted to sack Gary Barlow from X Factor'