The problem, it seems, is that gmaptogpx draws too straight a line between points. This can be seen by
- Draw a route on Google Maps
- Create a GPX using GMapToGPX
- Upload the GPX to maptogps
To fix this
- Draw a route in Google Maps
- Create a GPX using gpsvisualizer (copying the URL of the google maps from the "link" window into the box
- Remember to select "Add DEM Elevation Data"
This is a much better fit.
Now use gpsies to convert to a .crs and import to the Garmin.