There are pre-built free open street maps for Garmin bike computers. No software is needed, just download the map file, and copy it to your Garmin.
I posted detailed installation instructions in this thread.
I have a 705. ridewithgps.com has .tcx Course files that work great. Courses files are used with "Training" , not with "Where To"
And I've recently been using the "Only the cuesheet entries, not the entire track" version of the the .gpx file. I get good warnings at the turns, and it actually works correctly, not like the other .gpx full version, which often failed to route correctly. ( Garmin's software is often junky. Why would a full .gpx file with a breadcrumb trail and the turns fail to route? It would just skip whole sections of the route. Yet, the turns-only version works correctly...huh.)