The pair - who famously fell out after Robbie quit Take That in 1995 - have reunited to write and record a single, entitled 'Shame', which will be released on October 4.
The collaboration comes 15 years after they last made music together and follows months of speculation about whether the former bandmates would work together again following the renewal of their friendship.
'Shame' will feature on Robbie's upcoming greatest hits album 'In and Out of Consciousness: Robbie's Greatest Hits 1990-2010' which will be released in October.
The 36-year-old pop star admits he is very excited about the upcoming LP and his musical reunion with Gary, 39.
Robbie said: "It's incredible to listen to the album and realise that it's already been 20 years of making music and playing gigs.
"The great thing about the album is that it's not only a celebration of my past but also a bridge to my future. The fact that part of the future includes a name from my past makes it all the more poignant for me."
It was recently revealed the pair have become so close again that Robbie is planning to ask Gary to be best man at his wedding to Ayda Field.
A member of the 'Bodies' singer's family said: "Rob is very fond of Gary and considers him a solid rock in his life."