If Gundogan departs when his contract expires this month, he would leave not just with 14 trophies but with countless memories as well, having taught the all-conquering team that giving up was never an option.
The Germany international had missed the 2016 Euros with a dislocated kneecap and played only 10 league games before tearing the cruciate ligaments in his right knee and sitting out the rest of his debut season.
But even with the riches at City's disposal, it became evident that Gundogan was irreplaceable in Guardiola's lineup as he quickly became the heir apparent to Yaya Toure, the ageing hero of the pre-Guardiola era.
The unassuming and soft-spoken German never displayed the same flashy traits as his fellow superstars, instead quietly going about his business - overlooked by many but always an integral cog in the City winning machine.
Under Guardiola he became the ideal box-to-box midfielder and a reliable goal scorer with his late runs into the area, a silent assassin so often picked up by his opponents when it was already too late.
MR CLUTCH
Such was Gundogan's consistency that before Erling Haaland (22) arrived he was City's top scorer in the 2020-21 season with 17 goals, while his five-minute brace in the last game of the 2021-22 season helped City pip Liverpool to the title by a point.
Haaland may have scored 52 times this season but Gundogan has always been a constant threat. Now the team's captain, he has led from the front, his contributions defined by becoming the man for the clutch moments in City's historic treble bid.
In a tense title race where City had played catch-up for months, Gundogan scored twice against Leeds United as City maintained their advantage at the top of the Premier League in the home stretch last month.
He then repeated his heroics with a brace against Everton as they kept Arsenal at bay to win a fourth league title in five seasons.
Gundogan then stunned arch-rivals Manchester United in the FA Cup with another double, scoring his first by volleying home 12 seconds into the game. He was only denied a hat-trick by the offside flag.
THIRD TIME LUCKY
As reports of his exit at the end of the season gathered steam, with Arsenal and Barcelona linked to his name, Gundogan was unperturbed - a man on a mission to win the elusive Champions League crown that had escaped his grasp twice before.
He was on the losing side when Borussia Dortmund were beaten in 2013 by Bayern Munich and when City lost to Chelsea in 2021. Nevertheless, he insisted there were no special plans to ensure he was third-time lucky.
"We don't have to do anything special. I think the simplicity of doing our things makes us special," he had said before the game in Istanbul.
"It is about putting effort into the training sessions, preparing ourselves as well as possible."
Gundogan's simplicity is what Guardiola would miss the most if he loses not just one of his best players but also a friendly neighbour.
"We live in the same floor for many years so he is a close friend of mine," Guardiola said. "He knows what I think (about potentially staying at City)."
As for what's next for Gundogan, he is holding his cards close to his chest after finally lifting the Champions League trophy that City have dreamed of ever since winning trophies became a habit.
"I haven't decided my future yet."