The Take That star won't join fellow judges Nicole Scherzinger, Tulisa Contostavlos and Louis Walsh at the red carpet premiere tomorrow.
An X Factor spokesman confirmed that "personal circumstances" mean Barlow will miss the event at the Corinthia Hotel.
Barlow announced on August 6 that he and wife Dawn had lost their baby daughter Poppy, who was delivered stillborn.
"Gary is in no fit state to do it as all his fans understand," a source told The Sun.
Last year's winners Little Mix will perform a special acoustic set at the launch.
The X Factor starts on ITV1 this Saturday at 8pm.
Photo gallery - The X Factor series nine promo shots:
Copyright: ITV Thames/Syco