The 'Back For Good' singer and his bandmates Mark Owen and Howard Donald are hitting the road next year in recognition of the milestone, but despite speculation from fans that the former members could join them on stage, the former 'X Factor' star is adamant the shows are just for the three-piece.
Asked if all five will appear on stage, Gary told Heat magazine: ''No, I tell you why, we're very clear with each other - this particular project will be the three of us and we're going out next year on tour.
''As usual, we're always keen to say at any point if anyone wants to come back, it's an open door to any of the others.
''I mean, we were with Rob a couple of weeks ago and we had a great afternoon, but he's doing his own thing at the moment.''
Robbie quit the band in 1996, a year before they split, but declined to be involved when they reunited in 2006.
However, he came back to the band in 2010 for their 'Progress' album and tour and making the record was an experience Gary will always treasure.
Discussing his happiest memory of his time in the band, he said: ''We were in a studio in New York and all five of us - Rob, Jay, Howard, Mark, me - were writing 'Progress'.
''I just thought, 'Wow, we didn't even know each other when this started in 1989 and here we are, all these years later, making a record together.
''No one knew we were doing it - it was a complete secret - and I thought, 'I'm going to remember this forever.' ''
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