gpx.studio is a free online GPX viewer and editor which allows to:

visualize multiple traces in different colors and aided by many different maps (cycling, hiking, satellite, etc)
edit traces by adding, inserting, moving or deleting track points
reverse traces
add or change the timestamps of a trace
view, add, edit and remove waypoints
reduce the number of track points
view and rework the structure of the file
automatically add elevation data to traces if missing
merge multiple traces, extending time, heart rate, cadence, power and temperature data where needed
extract segments from traces and perform any other action while maintaining the segments' structure within files
save the result on your computer or to your Google Drive™ to get a shareable link and embedding code