You have to make a pincodes file which will be located in the following folder:
nn:nn:nn:nn:nn:nn as in your BT4 bluetooth mac address.
I got this from: HOWTO/Bonding â€“ BlueZ
In the pincodes file you will have to write on one line "MAC_ADDRESS (space) PINCODE"
So if your GPS device's MAC address was "00:11:22:33:44:55" and you wanted to make the pincode "1234" then you would write on one line:
After this you should probably edit your rfcomm.conf file so that the rfcomm connection is made automatically to the channel you specify when you start bluetooth.
Hope this helped!