What kind of GPX? An Activity or a Course? And why do you need to convert it? You can import GPX directly without converting it, into Garmin Connect, and if it is a Course, you can then send it from Garmin Connect to your device.
Because support is telling me that, while Connect will accept gpx files, it converts them and changes the data. Every time I upload a GPX file after having ridden about 2,000 vertical ascent, Connect takes off about 200 feet and changes the distance. They are telling me that Connect is really only intended to upload FIT files. From support, "Garmin Connect is directly used for fitness products like Edge cycling GPS models or Garmin watches like the fenix series."
If you record the ride with a Garmin device, it will be already in FIT file format. And the same goes for most 3rd party apps like Zwift, Rouvy, etc - they sync the activities in FIT file format.
However, if you need to convert an activity recorded in an app or by a device not capable of using the FIT file format, you can use an external tool for converting the GPX activity file. For example the FFRT should be able doing it: https://www.fitfilerepairtool.info/
And if you need to import courses (routes) to GC in FIT file format, the you can use for example plotaroute.com - it support the FIT file format.
