First, the KML must be structured exactly like one for POI.
A GPX is usually a tracking list, better limited to route / waypoints.
the KML for route planning is then called up via -> new route -> saved route
the right Way structure KML;
<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="http://earth.google.com/kml/2.2">
<Document><name>Name of the route</name>
<Folder><name>Waypoints</name>
<Placemark><name>first Waypoint</name>
..............................</kml>
Path:
iGO_xxxx\Content\Userdata\Route\my_Route1.kml (UTF-8)
Test KML with:
googleEarth and polsky POIConverter
+-+-+-+-+-+-+-+-+-+-+-+-+-+
an existing Route is saved as .... (default route 001), then under
iGO_xxx\save\profiles\01\users\00\routes\route001.dat
Spoiler: further explanation
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