I don't think Gündogan has to prove anything more to show that he's Barca quality. He has everything Barca looks for in a midfielder, a Xavi/Iniesta type of player, has succeeded in counter- and possession-based set-ups, and has shown his quality against great opponents and in big matches on the highest stages and he's absolutely consistent. His spine-injury has fully healed and it doesn't look like it's coming back to him ever again.
And there are so few offers for him. He went incredibly under the radar after his injury, most people still haven't realised that he's back to 2013 form. Everyone is busy with Mkhitaryan and Aubameyang.