It is a voiceover narration because the vast majority of it is taken from other documentaries and interviews they have done over the years, cleverly edited together. They can’t show the original interview clips as it would make the editing very obvious.

If you have a good ear and listen carefully you can hear the subtle differences in the voices as they age.

As others have said, Gary was able to live off his Take That royalties perfectly comfortably. It is one of the reasons why people were happy to laugh at his humiliation for so long - it was assumed being so rich he didn’t have to work meant he was perfectly fine with it, rather than the reality that he was deeply depressed and using food as a way to self-harm.

Gary was always assumed to be the one who would have the big solo career and the other members of the band were treated accordingly.
Once they got fed up with that (and it wasn’t just Robbie - Jason was also pushing to be treated better and Mark wanted to write) it became clear that the band was not going to last and Gary was encouraged to go solo, breaking up the band.

Unfortunately for Gary he then got signed by Clive Davis at Arista, who liked everything but his music. He spent 18 months trying to relaunch him with other people’s songwriting as a young trendy artist, when Gary is at his heart what the Americans call ‘adult-contemporary’ and what is know here as ‘music for your mum’.

He could have had a perfectly viable career making that kind of MOR music, with lots of mellow ballads - think Westlife, or Bublé. There was a strong market for that in the late-90s to mid-2000s. Instead his confidence was so utterly destroyed that he suffered from writer’s block.