Sports watches and bike computers record every activity as FIT, a compact binary format that general GPS tools cannot read. GPX is the open format that route planners, mapping sites and nearly every GPS app import.
Every format is read into plain GeoJSON features and written out from there, so coordinates, names and properties carry over wherever the target has a place for them. Only recordings with GPS positions can be converted; an indoor workout with no route is refused. Time and elevation are kept. Heart rate, cadence and power are not written, because GPX has no standard place for them; choose GeoJSON or CSV to keep them. Points become waypoints and lines become tracks; GPX has no areas, so polygon outlines are written as track segments. Elevation and time are kept.