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Not too sure exactly what you are asking, if you mean load parts (tiles) of an existing map in basecamp onto your device, this is possible but a bit of a pain. Used to be easier in mapsource ,but is rather more convoluted in basecamp.
I do not really understand your question about a gpx.
If it is a gpx track, this can be used on a device with no underlying mapping, if a gpx route file it will look for a routable map on the device.
The image is a gpx track on a device with no maps.
I have an etrex 30 and use it for hiking and cycling. Whilst not specifically a cycling gps it has ant+ therefore enabling things like heart rate monitors etc. to be used.