Thanks, Feanor.
If I delete the .kml suffix (leaving the .gpx), then open it in Text Edit, I get:
<?xml version="1.0" encoding="utf-8"?>
<gpx creator="mapy.cz" version="1.1" xmlns="
http://www.topografix.com/GPX/1/1" xmlns:xsi="
http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="
http://www.topografix.com/GPX/1/1 http://www.topografix.com/GPX/1/1/gpx.xsd">
<wpt lat="49.7445420921" lon="13.3757525682">
<name>001 Start</name>
</wpt>
<wpt lat="49.7444663197" lon="13.3801728487">
<name>002 Anglické nábřeží</name>
</wpt>
<wpt lat="49.744489789" lon="13.3801902831">
<name>003 </name>
</wpt>
<wpt lat="49.744489789" lon="13.3801902831">
<name>001 Start</name>
</wpt>
<wpt lat="49.7454768419" lon="13.3819873631">
<name>002 </name>
</wpt>
<wpt lat="49.7454768419" lon="13.3819873631">
<name>001 Start</name>
</wpt>
<wpt lat="49.7444663197" lon="13.3801728487">
<name>002 Anglické nábřeží</name>
</wpt>
<wpt lat="49.7433927655" lon="13.3798496425">
<name>003 Americká</name>
- which, when loaded on to the Garmin, does appear in 'Courses', and looks much as I would expect.
Do you recognise the above as a .gpx file?