Think it's just a good time and opportunity.
Gundogan's stock is the highest it's ever been with us. He's 30, and not guaranteed to keep this form up. Each over 30 really hurts his transfer value badly, and now is probably the last chance to get a good fee for him. If we don't sell him now, then you really then having to hold onto him until retirement/headed to a retirement league.
Aouar is a midfield fantastic talent, a guy I can see fitting Pep's system like a shoe. At 22 we could get at least a decade out of him, and he hasn't even hit his prime - who knows what the extra development under Pep at this younger age could for him as well.
The French league and teams are in the pits financially, so we could probably get him at a relatively knock down price. Playing and ambition wise, I think he'd jump at the chance to join us too.