TCX is a fitness-specific format that plain GPS and map tools do not understand. KML opens in Google Earth and Google My Maps. TCX is at home in workouts with laps, heart rate and cadence; KML is the usual choice for places, routes and tours viewed in Google Earth.
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. Altitude and per-point timestamps are kept; a line with timestamps is written as a Google Earth track that can be played back.