Even if Gundogan leaves Man City at least Pep Guardiola will look for other options, it's just that Gundogan contribution to Man City is quite significant and without Gundogan I don't think Man City will have made it this far in improving their performance. To be honest, Gundogan, who was appointed as Captain, explained that his role was vital enough that the decision to extend the contract was the best option. Apart from that, Arsenal and Barcelona, who had been eyeing Gundogan for a long time, had to look for other players.
Ilkay Gundogan is the captain of Manchester City, so of course he's one of the most important for them. So far Ilkay Gundogan has made 304 appearances for Manchester City with 60 goals and 40 assists during his contract, so naturally he will attract interest from several other teams to sign him. I think at the moment Barcelona are ahead of Arsenal to sign Ilkay Gundogan, but I'm very unsure whether Xavi will actually prioritize that signing this summer.
Xavi needs a defensive midfielder, while Ilkay Gundogan tends to play as a central midfielder. So maybe Barcelona won't be that serious about signing Gundogan if they manage to get a better defensive midfielder. I agree Barcelona must have some experienced midfielders apart from having De Jong, Pedri, Kessie, Gavi and Pablo Torre. With Ilkay Gundogan I think Barcelona will have good stability in midfield after Busquets leaves.