You can save the cost of an adapter. As long as the GPS has a cable with wires designed to attach to 12 volts, you can strip them down, tin them with a little solder to stiffen them up, and the insert the stiffened wires into the BMW connector.
I did this on my GSA and on my 800 GS - no issues. Once the wires are inserted ( sorry, I don't remember the color codings on the BMW end ) a good wrap of electrical tape will hold the wires in place and weatherproof it. TA DA!
I cannot change the laws of physics....