Does anyone know if there is a way to load this as a custom map to a garmin etrex 20?
Can you open the kmz file in your Garmin software on your computer?
If you can, you may be able to save the file as a gpx file that can be uploaded to your etrex. The map itself wouldn't load but the routes and waypoints should.
In order to see topo info and detail you will have to load Garmin compatible maps into your unit.
This is mostly speculation on my part, as I use a different brand GPS and the software I use lets me save in different formats (.gpx .kmz)