Hi Alexander,
Here are some conversion that succeeded but in QGIS is not same.
Gpx → Osm
https://app.screencast.com/9BYewaRMFc1SG
Gpx → Gml
https://app.screencast.com/1RsdoDrcUmN3f
Gpx → Gpx
https://app.screencast…...TopoJSON format does not support 3D geometry, but input Gpx is MultiLineStringZ...support 3D? If yes, will be in May. Or you will support 3D, if yes...