I created the routes in my My Drive app on my Android tablet, and then synced them as tracks to my GO51. Both the My Drive App, and the GO51 are set to avoid unpaved roads, but a number of the tracks used them - I had some fun excursions on dirt tracks through fields, a forest, and on one occasion an orchard - and those were the ones I used. I ignored several that were totally overgrown, didn't exist, or just looked too iffy. It's not clear to me whether that happened because of a problem with the algorithm, or the map, but the routes would have been better if they had stuck to paved roads. Is this what you would expect ?
The other thing that puzzled me was that the route as planned in the App would have a given journey time, but syncing it to the GO 51 almost always increased the journey time - sometimes by only a few minutes, but sometimes by quite a lot, which was quite annoying.
I'm afraid I've long since deleted the tracks, so I can't send them to you.