No. The GPS you will not be able to hear over the speakers.
I recently got a Magellan 4250. (bluetooth works great BTW) But even if you plug the cable into the AUX, I did NOT hear it over the speakers.
(I Dont have BMW bluetooth option)
OP: If you have BMW bluetooth, why are you looking to get a GPS with bluetooth???
2013 328i Xdrive | AT | Glacier Silver | Black Leatherette | Htd Seats | Moonroof