Hello Pascal,
I have seen this feature on some sites that use the Leaflet library instead of OpenLayers (which is the one the plugin uses).
Honestly, i wouldn't know how to do this with the OpenLayers library, or if it's possible at all.
The OpenLayers and Leaflet libraries are quite different in useage, so it's not just a matter of building in an option into the plugin to support Leaflet - it would basically mean a complete re-write, or even writing a completely different plugin.
So - sorry, but this is not a feature you can expect in GPXTrackMap anytime soon.
cheers, Frank