If he wins a CL with Barcelona, you can make a legitimate argument for him as an all time great midfielder.
He would be in the class of Kroos and co. at that point.
4 CL finals with 3 different teams, 2 wins.
They are already around the same level mate. This is why I find rating individuals based on team trophies so annoying.
Even if Gundogan loses the final, I wouldn't hold it against him as a player because I have seen enough of him to know his capability.
If Gundogan stars in the CL final, then that'll definitely add to his legacy but there are 2 other possibilities:
Barca lose the final/semi but Gundogan plays very well.
Barca win but Gundogan is shite in the final.
Even without another CL, I'd have the 2 around the same level. Gundogan has more well rounded attributes and is more versatile but Kroos is an absolute expert in his limited skillset. I'd argue that Gundogan is more clutch than Kroos even though Kroos has won more.
I feel like trophies are the most popular metric for midfielders and defenders in all time conversations these days because you can't measure them by stats like you can with forwards.
Kroos has many CLs and the WC which Gundogan doesn't have which is why people consider him a level above.
Gundogan's CV is pretty good though. Won stuff with Dortmund and City. He hasn't really won anything with Barca yet.
As it stands, Kroos has a considerably better CV but as players they belong to a similar level.
I'll rate Ilkay Gundogan as one of the best midfielders of his generation if he can win stuff with Barca while shining in those victories.
But I'll never rate him as one of the best midfielders in history because he has never been the best midfielder in the world for a decent time span.
From the last 10 years, Modric, Iniesta and KDB are a level above Gundogan and it will remain that way.