I do not know what GPS you use. I have no BT experience.
If the GPS is a wifi
You pair the device with a wifi GPS by connecting the device to the access point that contains the GPS data (your router, your AIS
transponder, etc.) then you select the network as a data source in OpenCPN settings.
This should be very easy.
If you pair to a wifi GPS your life on the battery
will be severely shortened. But without GPS onboard the device you do not have other good choices (there are BT gps units but I have only seen one BT AIS
unit). I have never seen BT instrument routers (if such exist).