The 40-year-old actress plays Pepper Potts - the love interest of Robert's character Tony Stark - in the hit Marvel franchise and she believes no one could ever match his portrayal as the billionaire superhero because Robert, 48, has made the character his own.
Speaking to Entertainment Weekly magazine, Gwyneth said: "Personally, I think it would be very hard for someone to step into Iron Man, because it is so Robert.
"You've got pathos, you've got humour, you've got a facility with language and improvisation and an incredible pain underneath it all. It's difficult to replicate that."
Robert loves playing Tony - who he has portrayed on screen five times including in spin-off movie 'The Avengers' - and all the action scenes that go with it.
However, he is acutely aware he is getting older and might not be able to keep up with his younger co-stars, such as 'Thor' actor Chris Hemsworth, for much longer.
He explained: "I'm going to be 50 in a couple years. I caught myself the last few years losing sight of that because I work with young people and I see Hemsworth and I relate to that. But then I'm reminded, 'He is that; you are something else.' "