TCX is a fitness-specific format that plain GPS and map tools do not understand. GPX is read by practically every GPS device and fitness app. TCX is at home in workouts with laps, heart rate and cadence; GPX is the usual choice for GPS tracks, routes and waypoints.
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. XML that declares custom entities is refused, and TCX files are parsed without touching the network. Points become waypoints and lines become tracks; GPX has no areas, so polygon outlines are written as track segments. Elevation and time are kept.