I use the BC-GPSK and it does the job fairly well. However any GPS unit that is NMEA compatible will also work.
I've also used Garmin GPS devices in the past with success, however cables were cumbersome (one went to the scanner and another powered the GPS unit instead of using the GPS on internal batteries). The Uniden GPS unit makes for a neater cable setup.
I have an older Magellan 310 which I'd like to try with the 436. I've used it before with mapping software on a laptop, so I know it will send data OK. I also have a serial programming cable for the XT scanners which should feed the data to the 436. What I don't know is if I need a gender changer only or a cross over connector. Can anyone help me out?