Tools:
gpsbabel -i gpx -f track1.gpx -f track2.gpx -o gpx -F merged.gpx
l=""; for f in $(ls *.gpx); do l="$l -f $f"; done; echo gpsbabel -i gpx ($l) -o gpx -F merged.gpx;
gpsbabel -i kml -f file.kml -o gpx -F file.gpx
-x duplicate,location-x track,move=-1h-1s-x nuketypes,waypoints, -x nuketypes,tracks-x simplify,crosstrack,error=0.001-x track,packCombine Tracks, and then right click on the new project, “Save as GPX 1.1 w/o ext” -x track,stop=20201231235959, -x track,start=20201231235959