The 'Let Me Entertain You' star left the band for a second time in April 2014, only to be followed by Jason in September of the same year.
The 44-year-old singer was asked by Phillip Schofield and Holly Willoughby on UK TV show 'This Morning' on Tuesday (19.06.18) to confirm rumours the band are plotting a full reunion - with him and Jason joining Gary Barlow, Mark Owen and Howard Donald to mark their 30th anniversary in 2020.
He said: ''I can genuinely say I don't know. I'm up for it. But I don't know what their calendar says, what my calendar says. I don't know what their manager says.
''We'd all love to do it. I'd love to get Jason back involved, realistically I don't think so.
''If we could get Jay back it would be perfect. But we shall see, watch this space.''
In March, Gary admitted there was ''no chance'' of Robbie rejoining the band.
The 47-year-old singer was adamant that the part-time member is highly unlikely to want to perform or work on new music with his bandmates after Robbie said he'd only return if Jason did too.
Gary said: ''I wouldn't put money on that. Don't waste your money. I would say there's no chance.''
However, his bandmate Mark wouldn't rule it out, he said: ''I wouldn't say there's no chance, but I've always said that.''
The five of them last performed together in 2011 on the 'Progress Live' tour for the first time in 15 years, but Gary said he's happy continuing as a three-piece.
He said: ''We feel like we did that whole five of us in 2011, so we're alright for a bit. We're not sitting around waiting for people to come back.''