What comes out

A GPX 1.1 file with every track point: latitude, longitude, elevation and time. That is what a map, a route planner, Google My Maps or a print needs. The file is named after the one you dropped in.

What is left behind, and why

Heart rate, power, cadence and temperature. GPX 1.1 has no standard field for them; they live in the FIT file's record messages, and carrying them over would mean inventing a dialect other tools cannot read. If those numbers matter, keep the FIT — the editor reads FIT directly and prints the numbers from it.

Nothing is uploaded

The conversion runs in your browser. The file is not sent to a server, and nothing is stored when you close the tab. That matters more for a FIT than for most files: it starts at your front door and carries your heart rate.

Strava's archive

The bulk export from Strava is mostly `.fit.gz` files — the originals, compressed. Drop them as they are; each is unzipped and converted. If you only wanted one activity, Strava's Export GPX on the activity page is quicker — the export guide shows where it is.

The other direction

GPX to FIT — for loading a course onto a Garmin — is a different thing: Garmin Connect imports GPX directly and builds the course itself, so most people never need a FIT. If you want the file on the device without Connect, GPX to FIT writes a course file you can copy to the Courses folder.