Two iconic figures have spent over a decade competing for a standing at the top of the global game.
They have shared 10 Ballons d’Or between them and broken countless records while raising the bar of individual excellence to heights never seen before.
The latter has since moved on to Juventus, but the battle between eternal rivals continues.
Both men have reached the notable mark of 600 goals at club level, while chasing down more major honours with their respective clubs.
Their place in football folklore is already secured, but the debate as to who can be considered the best appears set to rage on – probably long after they have hung up their boots.
For Gundogan, though, there is only one winner, with the mercurial talents of Messi considered to sit above a fellow superstar who is merely “world class”.
The Germany international told DAZN of the Argentine forward: “For me he has always been on a different level to anyone else.
“Cristiano Ronaldo is the world class player after Messi, but for me he is not on this Galactico level of Messi.
“I don't think we'll ever get a player like him again, so we should watch him and enjoy him while he's still here. I'm a big fan.”
Gundogan is not the only one to fall into that camp.
Messi has continued to maintain his remarkable standards across the 2018-19 campaign.
He has broken through the 40-goal barrier for a 10th successive season and is only two efforts short of reaching 50 for the fifth time in his career at Camp Nou.
Ronaldo has also added to his medal collection, becoming the first man to taste title success in the Premier League, La Liga and Serie A, but he has missed out on domestic cup and European glory this term.