The GPX files created by the ESP-GPS are fine as of firmware 5.81bis.
They use GPX 1.0 and include the correct header.
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<gpx version="1.0" creator="ESP-GPS SW 5.81bis" xmlns="http://www.topografix.com/GPX/1/0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.topografix.com/GPX/1/0 https://www.topografix.com/GPX/1/0/gpx.xsd">
<trk>
<trkseg>
<trkpt lat="51.0919304" lon="4.5245247">
<ele>11</ele>
<time>2023-12-01T11:38:51Z</time>
<course>303</course>
<speed>1.88</speed>
<sat>31</sat>
<hdop>0.49</hdop>
</trkseg>
</trk>
</gpx>