Marcus has been working with Gary, who mentored him on the show last year, on his debut self-titled album, out on Monday.
Speaking to the Daily Star, Marcus said: "The first day in the studio was a bit intimidating.
"There were plaques all around the room from such artists as Tina Turner, Michael Jackson, Cher... which made me wonder: 'Am I out of my depth?"
He continued: "The studio was also where Take That recorded their first album – I was two when they had their first hit.
"Gary is the fountain of all knowledge when it comes to stories of the music industry. We’re always having a laugh.
"He winds me up by getting me to sing when the mic is off so I get him back by pretending to pour cups of tea on him."
Marcus, who releases his debut single Seven Nation Army this week, added: "Gary is a great role model, family man and amazing musician – and I think he’ll do X Factor again."