I think Gundogan can really adapt to Barcelona's system in a short time. Because there are significant level of similarities between Xavi and Guardiola's systems. They both like to play a ball possession focused football so I don't feel like Gundogan would have difficulties in keeping up with the tempo. Tiki-taka system will be the one he will need to adapt to maybe but I don't still expect him to have serious issues considering his skillset. Xavi wouldn't have wanted him in the first place if he thought he might have issues there.
It is good to have some experienced players in the squad for Barcelona but they just need to keep the number of them not too high. It wouldn't be ideal for Xavi's system.
Actually easy for Gundogan adapt with scheme in Barcelona because not drastically difference yet game rhythm with Manchester City and how relationship between Xavi Hernandez and Pep Guardiola. In Manchester City, Gundogan doesn't need time for adapting with Pep Guardiola scheme because Bundesliga game rhythm almost the same with Barcelona show with tiki taka and dominance with ball controlling.
Right now, Xavi Hernandez has many option with midfielder position and not matter after leaving by Sergio Busquets because Gundogan will be right choose for replacing on holding midfielder. But difficult or not for Gundogan get first place or main player in Barcelona because on holding midfielder has compete with Pedri, Franck Kessié and Frenkie De Joong.