Bayern will let Gnabry move imo since his contract will end in a year and could you imagine the losses of the club if they hold him and Lewandowski at the same time for the next season and they end up as free agents!
There is Real Madrid, Liverpool, Arsenal, Chelsea, ManU but also City which are interested in signing him. City wants him since he can replace both Jesus and Sterling who will leave the club this transfer window.
The "this is a business" coldness that Bayern has proved many many times before how amazing they are and how they know what they are doing. If they want to keep those players they will offer them something lower than what they worth, and if those players accept they will keep them, if not then they will let them go and get players who will be about the same levels and still underpay them, or not famous players at all and get them to be famous.
In the end, if Bayern is doing something about either losing players or getting a player, both of the situation would be beneficial for them and for at least a decade they have been doing great job at both of them.