Pep has always happily explained his tactical decisions.
He picked Gundogan because we'd lost to Chelsea with both Rodri and Fernandinho playing 6. In the FA Cup especially we'd not managed to break the back 5 down, and he thought the only way to do so was to sacrifice the DM for another creative player and hope the back 4 could cover Havertz, Werner and Mount between them.
Gundogan has played dozens of games at 6 for us, including for a 14 match winning run to win the 2019 title so he'd played the position.
He said all of this in his pre match interview before the Champions League final.