Combine several GPX files into one: keep each recording or day as its own segment, or join them into one line. Everything runs in your browser — nothing is uploaded.
A watch that stopped for lunch, a phone that split a long recording, or a multi-day route saved one day per file: merging gives you one file to keep, share or import into Strava, Garmin or the TrailSplits planner.
A GPX track is made of segments, and a segment is a stretch recorded without a break. Keeping separate segments puts every file's segments into one track as they were, so a map shows two days as two lines and no app thinks you walked in a straight line from your hotel to the next morning's start. It is the right choice for keeping a record and for most apps.
Joining into one continuous line is for devices that follow a single course. Every gap between files then becomes a straight line; the page shows each gap before you merge, and the distance it reports includes those straight lines, because the file does.
Track points with their times, elevation, heart rate and other extensions; waypoints; routes; and the first copyright notice found among the files. Points that repeat where one file ends and the next begins are written once. Use Sort by recorded time to put files in the order they were recorded; the list warns when a file starts before the previous one ends.
Your GPX files stay in your browser. Nothing is uploaded.