Gnarby found his second career in Bayern. He was considered as talented players but he could not get a position in Arsenal and has to leave to other clubs. The destination in Muchen is good for him to grow his career and he gets what he should have with his talent.
I always say the same thing, give me a million dollars per year and send me to Bayern, even at this age and weight, I will be a semi-decent player. Sure not at the level that is worthy of Bayern we all know this, but training with them for 2-3 years and making that much money to do that will make nearly ANYONE be good enough to have at least another 2-3 seasons (if young then 10+ seasons) football career.
Certainly talent plays a big role in here, the more talented you are the better team you will play for, but having 2-3 seasons of Bayern training could mean third division bulgarian league type of talent for sure out of someone who is not a football player right now like me. All in all Gnabry was already at Arsenal level of talent, and when you put German work ethic on top of that, and a good strategy to play him, it was only matter of time for him to become a star as well.