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…... The TopoJSON format does not support 3D geometry...conversion. We can try to delete this z-coordinate and see what happens...