Golden Retriever automatically requests the most accurate GPS coordinates available from the device (either the built in receiver or a Bluetooth connected receiver) when a new record is started. The app will then save those coordinates when the record is saved. Changes to your required accuracy can be made in the Required GPS accuracy menu item. Note: changes to these settings can cause extra time for Golden Retriever to obtain the more specific GPS coordinates.
On iOS all you have to do is pair the Bluetooth device to your iPad/iPhone and then GR will automatically start getting it's coordinates from the Bluetooth device. No further configuration is needed inside the GR app itself. GR will still say it is getting it's coordinates from the "Internal GPS" but you will notice the accuracy will have improved.
On Android you will have a button in the GR menu to Enable/Disable the app from using the external Bluetooth device. Make sure you have paired and connected to the Bluetooth GPS device before clicking the button or else you will get an warning message that there GR could not connect to the Bluetooth GPS device.