Hi Stuart - if you've bought a Montana with no maps at all in, I'd recommend you get the City Navigator maps for Europe from Garmin to start with (on an SD card) - that will give you all the functionality of a regular Zumo/Nuvi GPS with a huge database of addresses and points of interest etc. and if you adjust the scale of the maps to 'most' on your Montana you'll see there is a huge amount of dirt road detail on there too.

Once you've loaded the TET (or other) GPX breadcrumb tracks into there, they will simply overlay on whatever map/s you have installed.

If any of the 'road/trail' detail is missing on the Garmin CN maps, the breadcrumb for the TET will still show on the screen anyway, and you just navigate along it by keeping your cursor on the line until the coincides with the map information again.

Note that the TET GPX files are not 'turn by turn' navigation, but the digital equivalent of tracing over a map with a highlighter pen on a sheet of clear acetate, and laying it over whichever map/s you choose.

You can convert a GPX breadcrumb trail into a turn by turn 'route' using the Garmin software, but it really isn't necessary - and keep in mind once it's 'routing' it will snap to the roads it thinks are the fastest/shortest based on the installed maps, not necessarily exactly the way you might want to go via the original breadcrumb track.

In a nutshell, personally I would use the 'go to' button to find fuel, food, hotels, addresses etc. and let the machine guide you turn by turn in those instances; while when navigating predefined 'trail' routes such as the TET, just display the relevant track on the screen in a highlighted colour of your choice, and follow it that way.

Hope that helps...

Jenny x